I'm not a JW, but I know a lot of people who are and I've been to a few meetings etc so:
1) They observe a celebration called the Memorial of Christ's Death. Taken from the last supper, when the disciples ate the bread and drank the wine Jesus asked his followers to "Keep doing this in rememberance of me". The reason they don't celebrate christmas, birthdays, easter is that these have their origins in Pagan worship and therefore they don't see these celebrations as originating with God.
2)What Aimsmum said.
3) They don't wear any special robes or gowns. For their meetings and when they are on the ministry, they are required to wear smart clothes.
4) Again what Aimsmum said . Also at the memorial I mentioned earlier, during the ceremony they pass around unleavened bread and wine, but the don't drink or eat it, just reflect on it's important meaning.
I really hope this is helpful! I'm not feeling very articulate this morning
My parents are JW, I was brought up as one until I was 15.
Aimsmum and wakeywoo have just about covered it all.
The baptism is a very big thing for them.
As mentioned they are required to dress smartly, woman generally don't wear trousers to the meetings. If there is a meeting where only women are in attendance and they are required to say a prayer for the group they need to wear a headscarf.