February
February is also the only month of the calendar that, once every six years and twice every 11 years consecutively, either back into the past or forward into the future, will have four full 7-day weeks. In countries that start their week on a Monday, it occurs as part of a common year starting on Friday, in which February 1st is a Monday and the 28th is a Sunday, this was observed in 2021 and can be traced back 11 years to 2010, 11 years back to 1999, 6 years back to 1993, 11 years back to 1982, 11 years back to 1971 and 6 years back to 1965, and will be observed again in 2027 In countries that start their week on a Sunday, it occurs in a common year starting on Thursday, with the next occurrence in 2026, and previous occurrences in 2015 (11 years earlier than 2026), 2009 (6 years earlier than 2015), 1998 (11 years earlier than 2009) and 1987 (11 years earlier than 1998). This works unless the pattern is broken by a skipped leap year, but no leap year has been skipped since 1900 and no others will be skipped until 2100.