The upcoming year will be a leap year in the Chinese calendar, which means that it has an extra month, not just an extra day. In Chinese, the calendar is often referred to as 農曆 (agricultural calendar) because the months while lunar in nature, are pinned to certain solar terms to help plan crop harvests.
This means that unlike in the Gregorian calendar, we can't just slap an extra month at the end bc it would screw up which months are in which season. Instead there's a complicated system...