The long-term average should be once a year, since there are seven 31-day months in a year.
If your period starts on sunday, you can start the pill that day.
In order for a month to have it's thirteenth (13th) day land on a Friday the month most start on a SUNDAY.
Several months can start on a Sunday, depending on the year. For example, January, March, May, July, August, October, and December can all start on a Sunday in certain years. You can determine which month starts on a Sunday in a specific year by checking a calendar for that year.
Friday the 13th only occurs when the first day of the month is a Sunday.
Sunday
To determine the first weekend of the month, start by identifying the first day of the month and then find the nearest Saturday or Sunday. If the first day is a Saturday, that day is the first weekend. If it’s a Sunday, the previous day, Saturday, is considered the first weekend. If the first day falls on any other weekday, the following Saturday will be the first weekend.
December is a whole month, Sunday is just a day so 'December" can't fall on any day of the week as there are 4.25 weeks in the month. If you we're really asking how often Christmas falls on Sunday, since the millennium Christmas has so far been on a Sunday in years 2005 and in 2011; and in 2016 Christmas will again be on Sunday.
sunday sunday
Friday the 13th only occurs when the first day of the month is a Sunday.
Take your first pill the Sunday after you get your period, even if you are still bleeding on that Sunday. You don't necessarily have be a Sunday starter, I just find it easier.
the day has to be a sunday
The second Sunday in May.