S = Solemnity, the highest form of celebration for major church events such as all Sundays, Christmas, Ascension, Assumption, All Saints, Immaculate Conception, St. Joseph. Solemnities include chants, music, gloria and creed.
F = Feast, the next level of celebration include certain feasts of the Lord such as the Transfiguration, the apostles. Such celebrations call for more chant, music and the gloria.
M = Memorial of a particular saint which the church is obliged to celebrate. Other Mass prayers cannot be substituted.
m = memorial of a particular saint which is optional to be celebrated or to celebrate prayers of other Masses that day.
f = ferial, a day in which no particular saint or feast of the Lord is attached to the Holy Mass. The priest is free to offer any votive Mass for the needs of the people.