Ustasa, ustanik, rebel, rebels against a corrupt regime. Croats with historical knowledge celebrate the 10th of April, as it was then when Croatia's independence was declared. 
The Ustasa were dressed in black (hence the name 'black legion'), not like politicians in blue suits, who actually think about their image. Did you know the color blue is also used by banks? This is because the color blue (color psychology: subliminal messaging) gives people the idea that they are trustworthy. 

(source) About many cultural activities in the NDH it was forbidden to write about. There was only one truth, Tito's truth!
Since little or nothing is spoken or written about the educational successes in the Independent State of Croatia (NDH), many people think that there were none. But, in the NDH cultural life was exceptionally rich. On July 15, 1941, the Yugoslav Academy of Sciences and Arts in Zagreb received the correct name - Croatian Academy of Sciences and Arts and continued to work on the educational and scientific field, publishing several books of its works. Until 1941, under the old name various collections were published like "Sources for the Oldest History of Croatia"(Izvori za najstariju povijest hrvatsku)(Rački), "Monuments from the era of Croatian rulers"(Spomenici iz doba hrvatskih vladara) (Bulić and Jelić), "Croatia before the year 1102" (Hrvatska prije godine  1102)(Rački), "Dubrovnik Historical and legal monuments, old Croatian writers" (Dubrovački povijesni  i pravni spomenici, stari hrvatski pisci)(20 books)," Dictionary of Croatian Language"(Rječnik hrvatskog jezika)(12 books), etc. The Academy published its annual anniversaries, as well as the proceedings and articles entitled "Rad(Work)", which were published until 1941, with a total of 271 books, and 7 for NDH. Through the Academy there was a library developed (90,000 books, 1942) and Strossmayer's gallery (700 valuable paintings and sculptures by domestic and foreign artists). Oriental collection with about 2,700 statues, devices from the Turkish era, a Glagolitic collection and Archaeological Museum. The Academy also published an international magazine in the French language................ Many museums and libraries were open at the time of NDH. In 1943, Croatia won the European Football Championship and achieved numerous successes in tennis, boxing and other sports. There was much more, but by the Partisans and Communists coming at power, led by Josip Broz Tito (who is still an honorary member of the HAZU!) books and newspapers had to be thrown into the trash or burned.  Most of those who were at that time active on a cultural scale, from visual arts, music and other artists, journalists, writers etc were imprisoned, and not a small number of them killed without trial. Those who managed to escape abroad were able to save themselves, but there were some whose bloody (UDBA) hand of Marshal Tito unfortunately did reach. (More than 60.000 Croats are killed after WWII by the Yugo regime (source))
The media and historians have had to hide the truth about NDH until the beginning of the 1990s, or write down only negativity and even a large number of lies, for which no one was held responsible till this very day, as well as to the atrocities committed by the Partisans (KOS, OZNA ...) over the innocent Croatian people. - Mladen Pavković, president of the Homeland War Veterans' Association91 (UHBDR91.)

What is needed, is the truth. Many documents like about Jasenovac are in Belgrade, and as long as myths are told that Croats put Serbian babies in ovens and more such nonsense, no wonder anything related to Ustasa makes the Serbian blood boil. Absolute and unnecessary cruelty especially against the weakest ones such as children, is that the Croatian people the Ottomans also battled against? The Turkish philosopher Kjafii in his book "Nizam ul Alem"("the Order of the World ") writes how more than anything he appreciates the Croatian knightly spirit, which will never hurt the unarmed or captured enemy, and the promise given will always be fulfilled. 
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Croatia declares independence - Apr 10, 1941. 
Thinking about 10 April> there was no Jasenovac in April of 1941, nor the persecution of Serbs or jews (source)

Text dr. Vladko Maček from word to word:

"Croatian people! Colonel Slavko Kvaternik, leader of the nationalist movement in the country, today declared a free and independent Croatian state on the entire historical and geographical territory of Croatia, and assumed power. I call on all the Croatian people to obey the new government. I call on all the supporters of HSS, who are in administrative positions, all district councilors, mayors and councilors etc, to sincerely cooperate with the new national government."

Even the Yugoslavs must admit that dr Vladko Maček was not a fascist. Maček and Kvaternik proclaimed the NDH.
Maček was even offered to become the leader of N.D.H. eventhough that he was far from the strongest party (HSS), but he refused.
Most of the period N.D.H. he spent in house detention in Kupinac and even in the Jasenovac camp (which he somehow managed to survive, which makes him a superhuman if we believe the Jasenovac myths)
Vladko Maček died on May 15, 1964 in Washington, Tito had expelled him from Yugoslavia.

MAIN ARGUMENTS AGAINST 10 APRIL CELEBRATION source

Croatia should not celebrate on April 10; "because Ustashas have killed many innocent people, including the Serbs"
Answer: Who didn't in WW2? All the armies did so. To make this argument worthwhile, you should find an army that did not patricipate in massive crimes. Armies don't shoot with flowers and fairy dust.

"because it was dictatorship of Pavelic?"
The Republic of Croatia still is not 100% free, even proven on 13.04.2018.

"because Pavelic sold Dalmatia to the Italians?"
False, see Treaty of Rapallo (1920) source

"because the foundation of Croatia is anti-fascism?"; that term did not even exist in 1941, even among the Partisans

Let's compare that with 'Mother of Europe', France.

The French Independence Day is July 14 and then the most famous parade in the world of this type is held. 

France should not celebrate July 14 because Napoleon's army has killed many innocent people, including the Serbs.
>France should not celebrate July 14 because it was dictatorship, first Robespierre, and then Napoleon.
>France should not celebrate July 14 because Napoleon sold the United States to Louisiana.
>France should not celebrate July 14 because its foundation is not anti-fascism.

CONCLUSION: History is complicated and there is never anything in it black and white. 
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There is another thing to remember on April the 10th. On that day, in 1947 in Split, the young Croat patriot Frane Tente removed the Yugoslav flag from Marjana (a hill that rises over the city). Instead of the Yugoslav flag on Marjan, a Croatian flag of 18 meters was set! This extremely brave and patriotic act was a protest of young Croatians against a violent and criminal communist regime that was conducting violence, terror and death throughout our Croatian homeland. 

One of the participants of this extremely brave action, Frane Tente, was killed a year later in Lepoglava in the age of 20. He is one of the thousands of Croat men and women who have paid for their patriotism and civic courage by being tortured to death, while opposing the inhuman and demonized communist regime.
On 10 April 1947 this group of young men and girls decided on a protest action; Ivica Bavčević (22), Nikola Pensa (22), Jelka Betica (42), Vlaho Zelinak (45), Borica Jonić (20), Ruža Anić (19), Katica Šanić (20), Jakov Kirigin (19), Tomislav Karaman (21, later priest and general vicar of the Split Archdiocese), Vjekoslav Matijević (20) made a courageous act the communists could not imagine that could happen in Croatia, that had been under terror for two years facing numerous tortures, persecutions and killings as "the enemy of the people's anti-fascist regime". Tente was sent to prison, was tortured and died.  source

 
Dishonoring Tito's victims, Ivo Josipović, the Yugoslavian fascist, liar and British servant repeats communist lies about the NDH.
In 1944 NDH returned all the occupied parts of Croatia, but Ivo like most communists, don't care for truth but propaganda to make yourself and your ideology look better.

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Pamphlet from Moša Pijade how to make partisans and rebels out of peaceful people and create the myth about Jasenovac! source

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Another reason to remember 10 April... 1992, the fallen Homeland defenders in Vukovar/Kupres