“When is United Nations (UNO) Day celebrated?”

"United Nations Day is observed every year on October 24th to mark the founding of the UN in 1945 when its Charter started working. The phrase United Nations (UNO) Day is a proper noun naming the specific day. The verb is celebrated is in present simple tense showing a repeated yearly event. You could also ask On which date is United Nations Day celebrated or When is the UN Day observed but the meaning is the same."