The name is reminiscent of an advent calendar, which traditionally has a labeled door behind which is a piece of candy, small toy or other trinket. These calendars are intended for young children to remind them of the period of advent.
If you are religious, look upon this as a similar thing which reminds you of the season, building in anticipation of the final day.
If you are not religious, it's just a word with no special meaning other than the time of year. Same goes if you are not of a Christian religion.
Because advent is a common term for the 4 weeks before Christmas even in non religious contexts. And that is how it's used in this non religious project.
> if this is a religious project, it should be clearly marked as such.
What an utterly strange proclamation. On every level: the notion that speech should not be free, the notion that users cannot be trusted to figure out what the project is about if it's not "clearly marked" in this way, the notion that there is some harm that can be done by such a "religious project" or a "religious word," whatever that even means.
https://news.ycombinator.com/newsguidelines.html