In the ordinary form of the Roman rite, the celebration of the Solemnity of the Ascension of our Lord may be transferred to a Sunday if it is not observed as a holy day of obligation in a certain region. I find this rule strange because the Scriptures say that the Lord ascended into heaven on the 40th day. If the Ascension is celebrated on Sunday, it would not be the 40th day but the 43rd. I may sound very petty but I have just been thinking that with regards Christmas, there is no rule that allows us to transfer the celebration of this feast to a Sunday if this were not observed as a holy day of obligation in a certain place. And yet, we all know that the date of the feast (December 25) is not indicated in Scriptures (unless there is something from Apostolic Tradition that I am unaware of). The Ascension event is different. The Acts of the Apostles really mention the 40th day.
Of course, in the extraordinary form, the celebration of an external solemnity on Sunday is allowed. However, even with an external solemnity, Ascension is still celebrated on a Thursday, which is the 40th day. Besides, the novena to the Holy Spirit begins on the Friday after Ascension Thursday. Even if the Feast is moved to Sunday, the novena should still begin on the Friday before that Sunday in order to have complete 9 days for the devotion (or else it could not be called a "novena"). Sometimes, I am amused by people who say, 'We will have a 7 day novena.' I find it strange that the novena to the Holy Spirit should begin before Ascension is celebrated. Did not Our Lady and the Apostles begin praying after the Ascension?