4 years
The month of July has 5 Saturdays and 5 Sundays zerotimes in 2012.
Over the very long average, once for every seven years. And it's "often".
About twice each year
Every 5, 6, or 11 years, depending on how the leap years fall. August had this configuration in 1982, 1993, 1999, 2004, and 2010. Next three times will be in 2021, 2027 and 2032.
4 years
Never.
October 2011, December 2012, March 2014 for starters. In other words, how often does a 31-day month begin on a Saturday? Fairly often.
The month of July has 5 Saturdays and 5 Sundays zerotimes in 2012.
Over the very long average, once for every seven years. And it's "often".
The odds are exactly 14.5%, which makes the average frequency once per 6.9 years.
April is not a month in the Hebrew calendar, and the Sundays in any Hebrew month could easily change from year to year, just as they often do for any month in the civil calendar.
March
ummm...never.
About twice each year
every 823 years.
Whenever December begins on a Saturday, which happens 4 times every 28 years, in a 6-5-6-11 pattern. December had this calendar in 1984, 1990, 2001 and 2007, and it will recur in 2012, 2018 and 2029.