Every 7 years on average. There are only 14 possible calendays (first day of year starting on each of the 7 possible days of the week, with and without leap year)
So on average a calenday will repeate every 7 years however due to year a callendar will either repeate in 6 or 11 years)
A callendar for 2010 can be reused during
2021, 2027, 2038, 2049, 2055, 2066, 2077, 2083, 2094, 2100, and 2106
It will repeate in 11 years twice in a row than in 6 years
October 2010
About twice each year
4 years
A month always has 28 or more days, so as there are 7 days in a week there are always at least 4 Saturdays and four Sundays in a month. As most months have more than 28 days, there can be up to 5 Saturdays and up to 5 Sundays in a month.
The month of July has 5 Saturdays and 5 Sundays zerotimes in 2012.
It has 5 Fridays, 5 Saturdays and 5 Sundays.
October 2010
2021, 2027, 2032, 2038, 2049, etc.
March
ummm...never.
About twice each year
every 823 years.
Never.
There is an internet hoax going around claiming that certain occurrences of five days in a month occur every 823 years. For examples, in October 2010, there were five Fridays, Saturdays, and Sundays. However, this occurrence occurs at the most every 11 years. The next time five Fridays, Saturdays, and Sundays occurs in a month is March 2013. The next time it occurs in October is October 2021.
March is the only month in 2013 that meets those specifications.
4 years
According to my desktop calendar, July will have five Fridays and Saturdays (and even some Thursdays) in 2016, 2021, 2022, 2027,2032, 2033, etc... If you look at http://www.timeanddate.com/calendar/monthly.html?year=2021&month=7&country=1 you will see that 2021, 2027, 2032 do not have 5 Sundays. The other years 2016, 2022, 2033 do have 5 Fridays, Saturdays, and Sundays. You are little confused since your first parahraph discusses Fridays and Saturdays, in which case 2021 and 2022 do indead have 5 Fridays and 5 Saturdays in July. In paragraph two above, you begin talking about Sundays and which years in which July will 5 each of Fridays, Saturdays and Sundays. Every month with 31 days will have 5 instances of the first 3 days in the month. This will repeat in a 5-6-5-11 year cycle and then repeat. So it should be 2016, 2022, 2033, 2039, 2044, 2050, 2056 - don't forget to consider leap year which will cause a 1 day shift.