The 1-st of April is not a national holiday. Shops & offices are opened, so you can come to your work, school or university and play a trick with your friends.
The day doesn’t commemorate any historical events. It’s just time for people to laugh and smile more than usually.
Nowadays, people tend to be busy and bored of their lives. On the 1-st of April they can let their hair down and relax even being at work. One of the best things about it is that everyone get involved in the celebration, even the calmest people join in playing jokes on their friends and colleagues. What I object to is playing cruel jokes wich can offend or injure somebody.