Every pre-Novus ordo missal I have from 1850 to 1967 all call the feast day Ascension Thursday and the following Sunday, First Sunday after Ascension. When the Novus Ordo changed the feast day to Sunday, I for all these years used it as a joke to show how they are a counterfeit church; "Ascension Thursday will be celebrated this Sunday".
Now the joke is no longer a joke, for many SSPX chapels have taken up the banner of "Ascension Thursday will be celebrated this Sunday" (What will they call the First Sunday after the Ascension?).
What is the big deal about Ascension Thursday? Is it a celebration of a miracle, Our Lord Jesus Christ ascending into heaven? No it is not. Ascension Thursday is a celebration of what Adam and Eve and all of the just since the beginning of man's existence awaited for 5000 years in Paradise of the Patriarchs, the re-opening of the Gates of Heaven by Jesus Christ. Ascension Thursday is the day that all the just awaited for 5000 years! Could there be a greater day for mankind? Ascension Thursday is man's prize for all of Christ's sacrifices as a man.
Now, the SSPX has announced that "Ascension Thursday will be celebrated this Sunday".
Catholics through history have given up their lives to horrific deaths (by burning by fire, boiling in oil, beheading, lances, arrows, disemboweling, firing squad, skinning and more) rather than give up the faith. The SSPX has given up Ascension Thursday for a bowl of pottage.