Okay kiddo, so a long, long time ago the Ancient Greek people didn't have smartphones or computers or even paper calendars like we do today. Instead, they used a special kind of calendar that was based on the moon and the sun.
So, the Ancient Greeks used two different calendars, one called the lunar calendar and the other called the solar calendar. The lunar calendar was based on the cycles of the moon, which would go through all of its phases in about 29.5 days. The solar calendar was based on the movements of the sun, which we still use today in our Gregorian calendar.
The lunar calendar had 12 months of 29 or 30 days each, which added up to a total of 354 or 355 days in a year. This meant that the lunar calendar didn't match up exactly with the solar calendar, which has 365.25 days in a year. To fix this, the Ancient Greeks would add an extra month to the lunar calendar every few years, so that it would line up with the solar calendar.
But the Ancient Greeks also had other ways of keeping track of time. They would divide each month into three periods called "dekatria", which meant "ten days". They also had special days that were set aside for feasting and celebrating, like the Olympic games, which were held every four years.
So, in summary, the Ancient Greeks used two different calendars based on the moon and the sun, and they also had special ways of breaking down time into smaller periods and celebrating important events.