Once every 28 years.
In 2032 there are 5 Sundays in February, May, August and October.
The only way there can be five Sundays in February is if it is a leap year AND February 1 is a Sunday. The last time that happened was 2004, and the next times it will happen are 2032, 2060, 2088, and 2128.
February, May, August and October.
25 times
Every time February 1st falls on a Monday !
Exactly 14% of all Decembers have five of each of Fridays, Saturdays and Sundays.
There are always 4 Sundays in February, except if it is a leap year and the 1st of February is a Sunday. In that case there are 5 Sundays.
About twice each year
The odds are exactly 14.5%, which makes the average frequency once per 6.9 years.
In 2001 there were five Sundays in April, July, September and December.
April, July, September and December have five Sundays in 2018.
January, April, July, October and December in 2028 will have five Sundays.