A higher than average chance for a blockade by a unique version of the enemy in question. Ex: Bane Invader, matched, who knows the drops. During the period of time the Bane/holiday NPCs are around they can blockade a planet. They have a 1/1000 chance of blockading one planet (with a maximum of one Bane/holiday NPC blockade on any of your owned planets at a time; so if random chance would give you two at once, only one of those planets would be randomly chosen). As it gets closer to the holiday's date, the blockade chance increases until you have a 100% chance of a blockade on the holiday's date, maybe the chances of the blockade would be altered to be exponentially rarer the further from the holiday it is, with it being guaranteed to occur on the main holiday itself (Halloween in this case).

If that coding is possible, of course.

Looking at it, I would think exponential would be best (holiday date = 100% chance of a player getting a blockade, 50% the day before, 25% two days before, 12.5% three, etc. This would put 15 days before the holiday at a .006% chance of getting a blockade.

I also cannot stress enough that the blockade chance would be for the player as a whole, not for each of their planets.