There is a hierarchy of celebration of saints.
Solemnities are the highest, they are not optional, and they displace the normal Sunday liturgy in Ordinary Time, but not in privileged times (e.g. Sundays of Advent or Lent).
Feasts come next. Feasts are mandatory. Feasts of saints do not displace the Sunday liturgy in Ordinary Time so they are not celebrated on days when they fall on a Sunday regardless of the season, but Feasts of our Lord (e.g. Transfiguration, Holy Cross) do displace the Sunday liturgy.
Next are memorials. These are mandatory but are not celebrated in the years where they fall on a Sunday; moreover in privileged times (all of Lent, and the last week of Advent), they become optional that year.
The lowest are optional memorials. These may be omitted at the celebrant’s discretion. When I pray the LOTH, I generally omit them unless they are specifically of interest to oblates or Benedictines.
Lastly, any saint mentioned in the Roman Martyrology may be celebrated as a memorial on his or her day, or even at a higher level if the patron of a parish or diocese.
The General Instructions of the LOTH give specific instructions on how to celebrate the Hours according to each type of celebration. The General Instructions of the Roman Missal do so for the Mass.
For the Mass, for memorials, in general unless otherwise noted the readings are of the weekday. There is no Gloria nor Creed. Antiphons will be according to the celebration (in Gregorian chant, from the Roman Gradual from either the Propers or Common of the saint).
For Feasts, the Gloria is said/sung.
For Solemnities, the. Gloria and Creed are both said or sung, even in Lent or Advent.