The history of the calendar in Roman times is actually an entire topic to itself.

The pre-Julian calendar required fine tuning every year in winter to keep the rest of the months aligned with the seasons.

Technically not a difficult job to keep the calendar running smoothly and consistently, but the person in charge of the calendar in Rome was a politician, so they would play political games with the length of the year.

Caesar wanted a calendar that would run on auto-pilot to strip power away from those politicians.

By sheer coincidence when Caesar made his reform, during the the changeover of calendars while he was in charge, he got to rule over a 400+ day long year.