What events happen reliably each season in Dwarf Fortress?

Migrants do have a chance to come in any season, so I guess that would be your main worry. People dieing decreases the chances anyway though.

A goblin caravan might come if you happen to have friendly goblins around. However, given the likelihood that that's a goblin siege, they wouldn't be sending one anyway.

If it's a season other than winter, you won't get a caravan, but the liaison will arrive after the siege is over. (Goblins don't send a liaison, btw.)

The only thing that I can think of besides caravans and migrants would be crops, but there aren't any winter-specific crops, so you should be good there too.