Any common year that starts on Monday has twoFriday the 13ths: those two in this common yearoccur in April andJuly.From July of the year in this type of year toSeptember in the year thatfollows this type of year is the longest period that occurs without a Friday the 13th, unless the following year is aleap year starting on Tuesday, in which case the gap only 11 months, as the next Friday the 13th is already inJune.
In the (currently used) Gregorian calendar, along withSunday,Wednesday,Friday orSaturday, the fourteen types of year (seven common, seven leap) repeat in a 400-year cycle (20871 weeks). Forty-three common years per cycle or exactly 10.75% start on a Monday. The 28-year sub-cycle only spans across century years divisible by 400, e.g. 1600, 2000, and 2400.
In the Julian calendar, the fourteen types of year (seven common, seven leap) repeat in a28-year cycle (1461 weeks). This sequence occurs exactly once within a cycle, and every common letter thrice.
As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. 25 cycles. The year's position in the cycle is given by the formula ((year + 8) mod 28) + 1). Years 6, 12 and 23 of the cycle are common years beginning on Monday. 2017 is year 10 of the cycle. Approximately 10.71% of all years are common years beginning on Monday.
Daylight saving begins on its latest possible date,September 30 in New Zealand andOctober 7 in Australia – this is the only common year where the period of standard time over the winter months lasts 26 weeks in New Zealand and 27 weeks in Australia (in all other common years, it lasts only 25 weeks in New Zealand and 26 weeks in Australia)
Daylight saving begins on its earliest possible date,March 25. This is the only year when First Sunday of Advent of the year preceding this type of year and start of daylight saving time are 16 weeks (112 days) apart, they are 17 weeks (119 days) apart in all other years.
