It's All in the Name by Vivien Teasdale

On 24 February 1582, Pope Gregory XIII proclaimed that the Gregorian Calendar would come into effect in October of that year. This would replace the Julian Calendar which had become ‘out of sync’ with what was actually happening to the seasons, which made it difficult to calculate the actual date of Easter. To make up for the discrepancy, the date leapt from 4 October to 15 October, thus losing ten days. Leap years are those that are divisible by four (eg 2024. Just missed it if you were thinking of proposing, ladies.) The quandary over whether to celebrate a 29 February birthday every year or only every fourth year did not occur until much later. The first Gregorian calendars simply had two days called 24 February. Why they chose that day, rather than having two 28 February days is a mystery. Years that are divisible by 100 are only leap years if they are also divisible by 400. The next one will be in the year 3000 AD. You may want to book early for the celebration...