Recommended Read: Why Julius Caesar’s Year of Confusion was the longest year in history

"For the two months of the year when there’s no work being done in the field, they’re just not counted," says Parish. The Sun rises and falls but, according to the early Roman calendar, no day has officially passed. "Which is where the complications start to come in."

https://www.bbc.com/future/article/20240227-how-julius-caesar-made-the-longest-year-in-history-and-brought-us-leap-years