Reference

The Easter season in the Roman Rite, also known as Eastertide, is celebrated as a festal season that lasts from Easter Sunday to Pentecost Sunday, marking a total duration of fifty days. This period commemorates the Resurrection of Jesus Christ and is seen as one continuous celebration known as the "Great Sunday" or the "Fifty Joyful Days."

The difference in duration is primarily rooted in liturgical practices and the adaptation of the early Christian community's celebrations. For instance, the first eight days of the Easter season form the Octave of Easter, celebrated with great solemnity. Throughout this Eastertide, various customs are observed such as flowering the cross and festive meals.

In some traditions, including the Roman Rite, the emphasis of the Easter season extends to commemorations such as the Ascension of the Lord, which is celebrated on the fortieth day after Easter, except where it has been moved to the Sunday after the traditional day (source: USCCB and Wikipedia).