You said you were going to make the map more festive and christmassy?

Not to be a pessimist but if TF2 ever did decide to accept this map, they'll probably make it only available when "Holiday Restriction: Winter" is turned on.

For the Invasion Update they did had an alien invasion version of 2fort, but when Meet Your Match was released they took it off the roster. Now it is only available on community servers.
Meanwhile there are many event maps which are skins of existing maps for Scream Fortress, but only available when Halloween Mode is on.
As for Smissmas itself, we rarely get new maps for Smissmas and when we do they're not festive. The last Smissmas map we had with snow was Snowy Coast and it felt more like a left over Invasion map.

But being optimistic, it's likely that if TF2 did decide to accept Snowfort (And I am hoping they do since who doesn't love snow levels?), they may choose the least festive of the two so that it may have more use throughout the year than just seasonal.

It's why I'm suggesting that maybe after you festivize Snowfort, may want to make it a seperate submission. That way TF2 can choose between two variants if they will decide to add your map. And even if they choose the least festive of the two, the other will still be available for the community to play with.