Ascension Day is the 40th Day of Eastertide (the season of Easter to Pentecost). As a community, we’ve been commemorating it for millennia, but in the 21st Century, no other festival in the Christian year is more important and less emphasized than the ascension of the Lord. Many grew up in faith traditions where it's not talked about much, if at all.
So Jesus went back to heaven. Duh. What’s the big deal?