The Gregorian solar calendar counts days as the basic unit of time, grouping them into years of 365+ and repeats completely every 146,097 days, which fills 400 years, and which also happens to be 20,871 seven-day weeks. Each “year” lasts exactly 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds.
To compensate for this anomaly, the Gregorian year is divided into twelve arbitrary months of irregular length, with no regular relationship among their lengths. English speakers sometimes remember the number of days in each month of the Gregorian year by the use of the traditional mnemonic verse Thirty days hath September.
A language-independent alternative used in many countries is to hold up the fists with the index knuckle of the left hand against the index knuckle of the right hand. Then, starting with January from the little knuckle of the left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month. The junction between the hands is not counted, so the two index knuckles represent July and August.
A similar mnemonic can be found on a piano keyboard: starting on the key F for January, moving up the keyboard in semitones, the black notes give the short months, the white notes the long ones.