Memorials are religious commemorations in the liturgical calendar of sainted people.
They are remembered by special prayers at Mass and the Divine Office. There are two types of memorials in the present calendar, obligatory, which must be celebrated, and optional, which may be celebrated.
Feasts are special days by which the Church gives honor to God, Jesus, the angels, and saints. They are divided into solemnities and memorials which along with Sundays of the year, Advent-Christmas, Lent-Easter, and ordinary time make up the liturgical year.
Solemnities are the highest rank of feasts. They may or may not be holy days of obligation. The number of holy days of obligation (days on which Catholics are obligated to attend Mass) is determined by the Episcopal Conference. In addition, the Conference can transfer a holy day to another date or dispense an obligation for that particular year. In the United States the holy days are the Mother of God, Ascension Thursday, Assumption of Mary, All Saints Day, Immaculate Conception, and the Nativity of Our Lord.
Certain diocese or provinces (collection of dioceses in a geographical area) have decided to move the Solemnity of the Ascension of our Lord from the fortieth day after Easter to the Sunday following that fortieth day.