What is the Catholic calendar, if I may ask, Fish?
Really what I'm referring to is more properly called a liturgical calendar.
Essentially, the secular year is divided up into religious seasons like Lent, Easter, Advent, and Christmas. During each liturgical season, some of the priest's vestments and altar paraments (cloths, dressings) have a proscribed color emphasis (purple in Lent, white and gold in Easter [Which is, yes, a specific Sunday, but also a seven week or so season following], as examples) that is only deviated from for specific feasts (For example, the feast of Pentecost and the color red supersede the usual use of green during Ordinary Time, which it otherwise is a part of) and for specific persons who have a color associated with their office (Cardinals have red hats, for example).
Each of these seasons also has a specific emphasis that is reflected in the mass. For example, in Lent, which is about the days leading up to Jesus' crucifixion, and lasts about 45 days, we see that story emphasized, as well as things with similar theological meanings, like the 40 days Jesus spent fasting in the desert at the beginning of his ministry and the Israelites' 40 years wandering in the desert. In Lent, the Gloria, an upbeat prayer or hymn of ancient providence that is normally part of the mass, is suppressed (Not used), and the word "Alleluia" is not used in the mass texts or hymns during that time. Other seasons have their own emphasizes and changes.
In this way, the Church goes through the cycle of Christ's life in each calendar year, and also goes through the eras of the People of God from creation to the end of time in another more subtle sense.
There is also a lectionary, which is a series of readings used on a given Sunday, and even a given weekday at every Latin Rite (Western church, not necessarily using the Latin language- would normally be in English or Spanish in the US) mass that day (With exceptions sometimes for optional or local feast days). These readings are carefully selected to match the tenor of the season, or specific feast days.
Instead of using the same readings every year, the Church has a three-year cycle of readings, so though the same seasons and feast days repeat on a one year cycle, you don't get the same exact readings at mass on the same day of every year (You get the same ones once every three years)- with certain exceptions, like Christmas (Christmas is actually an exception in several ways- first that it has the same readings from year to year, but secondly that it's readings are different depending on whether you go to an early Christmas Eve mass, a midnight mass, or a mass Christmas morning- the midnight mass readings are the best, although in the US a poetically lacking translation is used for the passage from Isaiah). If you were to take the 5th Sunday in Ordinary Time in the summer, though, and look at the readings at Sunday mass, they would match the 5th Sunday in Ordinary Time 3 years prior or 3 years hence. Of course, universal and local feast days can supersede the normal readings and colors, and sometimes because the date of Easter varies, there can be different permutations of just how many Sundays are in a season and just where certain feast days fall relative to Sunday).
Anglicans and Episcopalians follow a similar system. Lutherans do as well, but to a much lesser extent. And then some Methodists will sort of sometimes occasionally pay attention to a little bit of this stuff some of the time in some places.
By the time you get to more Protestant-y Protestants, like Baptists, Sunday is just Sunday and there are no special colors, customs, readings, or anything like that- it all becomes what the local pastor wants to have done and read and talked about. Catholicism is much more into a religious cycle of time that manifests in masses and other commemorations in tangible ways (As are select Protestant groups, but not all of them).
The Eastern Orthodox, and Eastern Catholic churches in communion with Rome, have a separate liturgical calendar, but of course with some overlap, because they come from different traditions, but ones with common roots as Latin Rite Roman Catholicism.
This is by no means a complete overview, just the general "gist" of things. It's far more complex when you really drill into it.
