Low Sunday - in contrast with the heights of Easter and the whole week following (Easter Monday, Easter Tuesday, Easter Wednesday, etc.)
There's even a special blessing before meals for Easter and the following 6 days -- practiced by Traditional Catholics -- "This is the day the Lord has made, alleluia. Let us rejoice and be glad in it, alleluia." You can say it in Latin as well, as we did at St. Thomas Aquinas (traditional) Seminary in Winona, MN.

Dominica in Albis deponendis - or, Put-aside-the-white (robes) Sunday, as this was when Catechumens, baptized on Holy Saturday, would put aside their white robes they had been wearing all week.

Whitsunday - see "Dominica in albis".

Quasimodo Sunday - The famous character "The Hunchback of Notre Dame" was named "Quasimodo" after the first word of the Introit for this Sunday. Many Masses are named after the first word of the Introit.

And, of course, if you're not Traditional:

Divine Mercy Sunday - If you're not Traditional, you want to erase all the above significance of this Sunday, and you think the world needs to remember more that Jesus is about Mercy (hint: THAT'S THE LAST THING the modern, libertine, do-what-feels-good world needs!!!) then you will call this Sunday "Divine Mercy Sunday" in between promoting St. John Paul II, speaking in tongues, and promoting Medjugorje.