The Gloria is a beautiful prayer that we sing towards the beginning of Mass, meant as a way to direct our praises to God in celebration of his coming as man 2000 years ago. We begin with the words of the angels in Luke 2, rejoicing that our Savior is born and has come to redeem us.
"And suddenly there was a multitude of the heavenly host with the angel, praising God and saying: 'Glory to God in the highest and on earth peace to those on whom his favor rests.'" - Luke 2:13-14 | NABRE
We don't always sing the Gloria though, such as weekday Masses and especially during penitential liturgical seasons, as well as most weekday Masses. For music ministers, this can become confusing, so when do we sing the Gloria?
Here's the simple answer:
Sundays in Easter
Sundays in Christmas
Sundays in Ordinary Time
I always check the title of the liturgy for the day, and if the title has one of the words above, then you sing the Gloria. For example, we do not sing the Gloria during Lent or Advent, but if there is a Feast or Solemnity during these seasons (such as the Solemnity of the Immaculate Conception), then you sing the Gloria. If there's a wedding during Advent or Lent (many parishes/dioceses do not allow weddings during Lent), you sing the Gloria. A few unique liturgies are Palm Sunday, when you will not sing the Gloria, but on Holy Thursday, Evening Mass of the Lord’s Supper, you’ll sing the Gloria since it is a Solemnity. Today, December 12, is the Feast of Our Lady of Guadalupe. Key word: "feast". Sing the Gloria! You'll notice other unique days throughout the week are called "Memorials", and it can be easy to confuse these with "Feasts" since these "Memorials" are typically used to commemorate certain Saints' feast days. Notice the title of the liturgy is "Memorial" though, and not "Feast", so we do not sing the Gloria on these days. As you use this rule of thumb, it's important to remember that we celebrate Christmas and Easter for 8 days, so we sing the Gloria throughout the week after Christmas and Easter.
If you want to double check, I recommend picking up a small book called an Ordo, which are published according to each diocese. They indicate a lot of useful information about each liturgy, such as should we pray the Creed, what color vestments should the priests wear, and should we sing the Gloria. Paulist Press also has an app called "Ordo", but you have to be sure to buy a new app every year. Most importantly, memorize the list above and wield it with confidence the next time you prepare music for Mass.