Adding a seasonal mod to the pack would either be silly or a bad idea.
It would be silly because to keep it from being a bad idea, they'd have to keep it in the pack year round, which would really screw with the biomes, weather, and keeping your water thawed.
It would be a bad idea if they include it seasonally because if players have ANY ChristmasCraft things in a chest anywhere, attempting to open the chest would cause a crash after ChristmasCraft is removed.