system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: This year, Hanukkah and Thanksgiving fall on the same day and apparently this won't happen again for another 80,000 years. Why is it so rare and why 80,000 years?

I assume it has to do with the lunar/solar calendar thing, but what exactly is going on?
gpt
You already said it. A lunar year is simply shorter, about 354 days, which is not the about 365 days the solar year is. Since they are based off different standards, the dates do not always coincide. There are corrections every few years for the slight differernces between the length of the day and year, which keep their months in certain seasons.