Just a little note for those who didn’t notice: we won’t be seeing another month like this month of this year for quite a long time again — one that starts on Sunday and ends on Saturday, comprising exactly four weeks.
Thankfully, someone already thought to ask my question before I did (although some of the people answering didn’t read the entire question). This one appears to be the most sound:
The last time this happened was 2004. It doesn’t happen every “so many years”. If it’s leap year obviously there cannot be a perfect 28 day month even if the first day is a Sunday.
Non- leap years push the dates one day ahead the following year. For example, if Jan 31 1 fell on a Friday this year, next year it would fall on a Saturday (perfect Feb 1 on a Sunday!). If this were to happen on a leap year however, next year Jan 31 would fall on a Sunday causing the perfect Feb not to happen.
So, a perfect 28 day February beginning on a Sunday and ending on a Saturday (exactly four perfect weeks) happens every six years until Jan 31 falls on a Saturday on a leap year; then we wait 5 more years.
There is either 6 or 11 years between. There is a random pattern of 6’s and 11’s. The long stretch is caused by Jan 31 falling on a Saturday in a leap year sending Feb 1 to start on Monday the next year as opposed to a Sunday if it were a non-leap year. So then we had to wait for the cycle to run through all days of the week again and until Jan 31 fell on a Saturday in a non-leap year.
Perfect February’s since 1970, next was 81, 87, 98, 09. Next one is due in 2015.