It has nothing to do with atomic clocks. Centuries ago, long before anybody knew about atoms or atomic characteristics, the smart people of the time figured out that the astronomical year ain’t exactly 365 days. By hours (somewhere around 1/4 day per year), it is slightly more. So, they decided to add an extra day, every four years, to get the ‘people’ calendar back on track with the actual universe. This causes traditional dates like Christmas, which normally advance by one day of the week each year, to advance an extra day of the week, e.g., Christmas was on a Sunday in 2022, then Monday in 2023; in leap year 2024, Christmas Day will ‘leap’ forward to Wednesday (instead of Tuesday).
Enjoy the extra day. It ought to be an international holiday, IMHO.