I can't remember if I've asked him in the past. We've been friends since high school, so it's been a long time. I'm not asking about Christmas, but about Advent.
I'm not asking about Christmas, but about Advent. Why would someone who hates Christianity want to celebrate Advent? Advent hasn't become commercialized yet.
How did we get from "atheist" to "hates Christianity"? Someone can doubt that God exists without hating Christianity.
Whether we're talking about Christmas or Advent, does it really matter? Advent calendars are popular even if not as loaded with marketing as Christmas. Many people enjoy having a few lights around to brighten up the darkness. They can borrow from Christian themes without being accused of cultural appropriation, unlike a festival such as Diwali.
My wife doesn't like the dark evenings this time of year very much. Right now in our living room we have a Christmas tree (lit up), a small ceramic Christmas tree (lit up), and a Father Christmas figure with a battery powered candle inside that lights up in the evening. Does being a Christian mean she shouldn't celebrate the rather secular figure of Father Christmas?
As to why your friend in particular does it, you'd have to ask him. I'm not sure why it's even noteworthy - it seems no different to people wearing a cross on a chain because it's a cool piece of jewelry rather than because they regard the cross as having any significance to their lives.
