You can get away with birthdays depending on the local elders. If they catch you doing Christmas or anything else religious then a number of things can happen.
1 - nothing. They ignore you, maybe spread some things locally and wait for you to resurface at the meetings before offering "counsel".
2 - they knock at the door, confront you and invite you to a JC. If this happens you 99% will be DFed and an announcement made.
3 - they view it as following another religion and therefore disassociation by your actions (one of the few valid reasons for this). An announcement will be made. In principle this is the correct action per org instructions but is not always followed as option 1 or 2 may be more convenient for them