I think you are confused. Ascension day is the Feast of Firstfruits....always a Sunday.
The Greek term Pentecost we use is the Feast of Weeks....Shavuot. That day is also always known.
It is the most confusing day there is, we always assumed it was 10 days before Pentecost because we also confused Pentecost with Shavuot.
However, if you look at the typology Shavuot is clearly the type of the ascension and according to the Old Testament principles God does things according to His appointed day. As a result many have gone back through the record carefully and it seems the account of 40 days up to ascension does not clearly say that it began on the day of resurrection. Rather it seems there may have been another week involved.
Now I feel it is very clear that Pentecost is not Shavuot, it is the feast of new wine which is 50 days after Shavuot. Another reason we confused this is we thought that 7 complete sabbaths is 49 days, however, a new moon sabbath is 2-3 days and counting the sabbaths from Passover you will have two new moon sabbaths, so no Jewish person would confuse 7 sabbaths plus one day as being 50 days.
So then does it make sense that ten days before Shavuot the Lord tells them to remain until endued with power and "when the day of Pentecost was fully come" they counted out the days as they were supposed to and yet completely forgot to mention Shavuot one of the three holiest days of the year?
Shavuot was when God betrothed Israel and depicts Moses ascending into the mountain with God. It is a very clear picture of the ascension.
Pentecost, the feast of new wine is a perfect depiction of the start of the church age. The New Testament ministry is the new wine, the church is the new wine skins, and the gospel is where everyone in the whole world hears us speaking the great things of God in their own language.
As for Ascension day always being on a Sunday that is because we view Sunday as the first day of the week, the day after the Sabbath. It is a distortion because we are trying to fit the Jewish calendar into the Gregorian calendar.