Advent is the period of four Sundays and weeks before Christmas (or sometimes from the 1st December to Christmas Day!). Advent means 'Coming' in Latin. This is the coming of Jesus into the world
I'm not the greatest at grammar, but I think the very first one is incorrect. It should just be a comma, then the second just a comma as well, but the last one is correct. (I have no idea, I am sorry if you get it wrong.)