A world calendar is like a special kind of calendar that includes all the days of the year, and makes sure all the months have the same amount of days (30 or 31), except for February which has either 28 or 29 days depending on whether it's a leap year or not. So, it's like a super-organized way to keep track of time which helps people all over the world stay on the same schedule.
Normally, a year is 365 days and 6 hours long, which means that every four years we add one extra day (February 29th) so that we can keep on track with the rotation of the earth around the sun. But, with the world calendar, we don't need to worry about that because every year has 52 weeks and an extra day called "Worldsday" which is kind of like a bonus day.
It was created to help people all over the world plan their lives and coordinate events no matter where they are. It's really helpful for things like organizing holidays or planning international events because everyone knows when they are and when they can expect them.
Overall, the world calendar is just a super-organized way to keep track of time that helps people all over the world stay on the same schedule!