Okay kiddo, so a calendar is like a big book that tells you what day it is and when important events happen.
The Byzantine calendar is a special calendar older than the one we use now, and it was used by people who lived a long, long time ago in a place called Byzantium (which was basically ancient Greece and Turkey).
Now, this old calendar is a bit weird because it doesn't quite match with how long a year actually is! You see, it has 12 months, just like our calendar, but some of those months have different numbers of days than they normally do.
It also starts on a different day than our calendar does, which can be pretty confusing if you're trying to keep track of things.
But the Byzantine people thought it was important to keep using their old calendar, even if it wasn't perfect. They had lots of important religious festivals and events that were tied to specific days on t his calendar, so they wanted to make sure they kept everything in order.
That's why they had special people called astronomers who would work to figure out exactly when each month started and how many days it needed to have. These astronomers would watch the sky for certain signs and use fancy math to make sure everything lined up just right.
Overall, the Byzantine calendar is a bit complicated, but it was very important for the people who used it. It helped them keep track of time and plan for important events, just like our calendars do today!