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.
Friday the 13th only occurs when the first day of the month is a Sunday.
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 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.
It was a Sunday.
The first of every month isn't called Mother's Day. Though, Mother's Day does land on the first of the month in some countries, it is only the first of one month, not every month. Generally speaking the first of the month is not known as Mother's day. Mother's Day and Mothering Sunday are both held on specific dates. In Great Britain Mothering Sunday is the fourth Sunday in Lent.