In our sanctuary, you'll notice that the table runner and bookmark on the Lord's Supper table in front of the pulpit have matching colors - and that those colors change from time to time. Why is that, and why is it important?
Over the past two millennia, churches around the world have developed a fairly intricate "church calendar" (the fancy term for it would be "liturgical year") based on key events in the Bible. At its best, observing the church calendar immerses us in the life of Jesus every single year.
Think of it this way -- we observe Christmas and Easter every year, right? (In fact, that's the only time a lot of people go to church at all; those people are completely devoted to the liturgical year.) So, the birth and crucifixion of Jesus. But what about all of the other events in Jesus' life that help us understand the meaning of Christmas and Easter? That's what the church year puts in context for us.
The website builder flockspot.io uses this graphic to explain to potential customers when and why the colors on their website might change. I find it to be an easy-to-grasp picture of the church year (ignore the proportions for the moment):
Part of the year focuses on "The Story of Jesus"
Advent. This is the "month" (4 Sundays) before Christmas. It is a time of preparation, with each Sunday focusing on the themes of hope, peace, joy, and love.
Christmas. This is the so-called "twelve days of Christmas" when we simply rejoice in the gift of Jesus Christ.
Epiphany. This word means "manifestation" or "appearing" and has been traditionally associated with the coming of the magi -- the appearance of Christ to the Gentiles. It has been given the arbitrary date of the Sunday closest to January 6. The following weeks emphasize the ministry of Christ, leading up to His Transfiguration.
Lent. This is the 40-day period before Easter (excluding Sundays) in which we focus on the price that Jesus paid for our salvation. It is a time for looking inward in repentance. Key days include Ash Wednesday, the first day of Lent, Palm Sunday, when Jesus triumphally entered Jerusalem the Sunday before His death, Maundy Thursday, when Jesus instituted the Lord's Supper, and Good Friday, when Jesus was crucified.
Easter. The 50 days leading to Pentecost are a time of celebration and rejoicing. We focus on the meaning of salvation and eternity, encouraging one another to share the good news.
Pentecost. Pentecost is the "birth" of the church, leading to the rest of the year, called "Ordinary Time". It is time for us to focus on our role as followers of Christ, building a strong and healthy church, and applying the life of Jesus to our own lives.
When put like that, the church calendar isn't so complicated after all!
This graphic is a little more precise, but it should not be daunting to you anymore:
The only dates actually mentioned in the Bible are Easter/Holy Week, and Pentecost. Everything else has been determined by church leaders over the years (so, your questions like "why is Lent 40 days?" and "why are there four Sundays in Advent?" always come with the answer "because a church leader thought that to be the best way to teach church members about the life and ministry of Jesus"). That also should help you understand why many church do not use the traditional church year, choosing instead to follow the secular calendar or a calendar of their own creation.
Here is one great reason to pay attention to the church year: it's based on the life and ministry of Christ. the secular calendar is not. So, while it's great to celebrate Mother's Day and Independence Day, let's remember as followers of Jesus that we want to be more shaped by Him than by the world.