A bissextile or leap year is one that has an extra day. It is a discrepancy in the calendar that can be explained by the fact that the Earth does not take 365 days to make one trip around the Sun. It takes exactly 365 days, 5 hours, 48 minutes and 46 seconds. These additional hours are rounded to 6, and the small difference accumulates and is added to the calendar every four years in the form of a new, 24-hour-long day.

The Egyptians already had a leap year every four years. When Julius Caesar arrived in Egypt, the Romans adopted the idea of the leap year and created a new calendar that was called the Julian calendar. This new way of keeping track of the days was not perfect, either, and a day had to be added to the calendar every 128 years.

It was Pope Gregory XIII who refined the Julian calendar: under the current Gregorian calendar, for every four years we have an extra day, except for years that are multiples of 100. Even so, to make the numbers balanced, there is an extra day within years that are multiples of 400. So, 1900 (a multiple of 100) does not have an extra day, but 2000 (a multiple of 400) and 2020 do.

Most people think that the day added to the calendar is 29 February, but this is not exactly true. At first, the day that was added was the 24th of February. That’s why it’s called bissextile, because it adds the bix sextus dies ante calendas martii (“sixth day before the first of March”). It wasn’t until the Gregorian Reform that the added day was changed to be the last day of the month (29 February).

Due to this, people whose Saint’s Days are between 24 and 29 February will see a small change depending on which calendar they look at. There are calendars that say that the leap year adds the day of the 24th of February and, therefore, no Saint’s Days will be celebrated on that day. In other words, according to these calendars, people who celebrated their Saint’s Day on the 24th in 2017, 2018, and 2019, will do so this year on the 25th of the month. But there are also calendars that add the day of the 29th. So, just like the rest of the year, they only put the saints that died on that day, like Saint Hilary and Saint Romanus.

Let’s see an example: the Catalan regional almanac “Calendari dels Pagesos” for 2020 said that 24 February was Saint Modest, Saint Edilbert, and Saint Primitiva’s day, while another regional calendar, the “Calendari de l’Ermità”, show no Saint’s Days on the 24th and celebrated these three saints on the 25th. According to this calendar, the Saint’s Days that are normally celebrated from the 24th to the 28th, were celebrated this year between the 25th and 29th of February. This is a unique situation that only happens once every four years.