Surprise party
A surprise party was a party thrown for a person without their knowledge beforehand. This was commonly done for birthday parties.

In 2370, Lieutenant commander Worf was worried that the crew of the  and particularly Commander William T. Riker might host a surprise party for his birthday. In at least five alternate quantum realities, Worf's concern proved to be well-founded, as such a party was indeed held. In one such reality, the birthday cake at the party was chocolate, but in another it was not. In another, Captain Jean-Luc Picard was able to attend the party, whereas in the others his duties had kept him from attending. In still another, a painting gifted to Worf by Lieutenant commander Data shifted position on the wall of Worf's quarters. In yet another, the contents of the painting changed entirely. In the prime reality, however, no such party was held, as Counselor Deanna Troi had talked Riker out of hosting the party, correctly suspecting that Worf would hate it. After she told him this, Worf instead invited her to share dinner in his quarters, ordering champagne from a replicator. ()

Miles O'Brien's birthday was in September, which caused his  confusion as to why people were acting suspicious around him in 2370. He thought they might be planning a surprise party, only he didn't know what else he had to celebrate. ()

External link 
 

Celebrations