Five Mondays in February: when was the last time? when's the next?

28 years ago.

I knew that without googling, because the calendar repeats every twenty eight years.

I am not as smart as @zenvelo and had to Google for my answer which uses 5 Sundays in their example and would assume this explanation would apply to Mondays as well. The link says it depends. Apparently the century year throws the 28 year theory askew to yield “the average is longer than 28” years.

@zenvelo is correct. The first year of a century is not a leap year unless it is divisible by 400. The year 2000 qualifies and was in fact a leap year, because it is divisible by 400, and so it does not disrupt the 28 year rule.

This site gives the day of the week for a given date. 28 years ago was 1988. If you enter 02/29/1988, you will see that it comes up as a Monday.

To clarify @zenvelo‘s point, Leap Years repeat every 28 years, and that is true. However, the non-Leap years repeat on 6—, 11— and 12-year cycles.

Check out ’‘repeating calendar’’ for the evidence.

@LostInParadise you were so close… we’re looking at the same site, but the page for “repeating calendar” shows the exact response to the question.

EDIT: I forgot about the years ending in “00”, which would normally be leap years, but are not. That means that the gap for some Leap Years to repeat may sometimes be 40 years.

Well done, @zenvelo.

I put a link to timeanddate.com in my question. I just couldn’t find the answer there.

In any year, the day of the week for the last day of February, leap year or not, is the same as 4/4 (April 4), 6/6, 8/8, 10/10 and 12/12. Knowing this helps to determine the day of the week for any day in case you do not have access to a calendar or a computer.

Okay, more calendar trivia (I can’t resist). The calendar works on a 400 year cycle. You might think that it should be a 7 x 400 = 2800 year cycle, except that, by chance, the day of the week after 400 years is the same.

This is easy to show. 365 has a remainder of 1 when divided by 7, so in non-leap years, we add 1 to the day of the week. For leap years, add another 1. There are 400/4 -3 leap years. We subtract the 3 for the 3 centuries not divisible by 400. That means that after 400 years, we advance by 400 + 100 – 3 = 497 days. which is divisible by 7, so the cycle repeats after 400 years.

There are 12×400 = 4800 months in 400 years and so 4800 times that there is a 13th of the month. Since 4800 is not divisible by 7, some days of the week will have to turn up more often than others. It turns out that Friday the 13th is the most frequent, though just by a few times.

The outstanding Jellies above nailed it. I wish I could add more other than an insistence that there is a Garfield joke in here somewhere just waiting to come out.