You may need to edit dlclist.xml in update/update.rpf/common/data to ensure the pack is active, although this is usually handled automatically by the game.
Rockstar frequently removes seasonal content from the base game files after the holidays to keep GTA Online streamlined. However, many Single Player modders (using tools like OpenIV) want the Christmas atmosphere year-round. If you deleted your game cache or installed a mod that overwrote the pack, you may need to re-acquire it. Update X64 Dlcpacks Mpchristmas2017 Dlc.rpf Download