How often in years in March 25 on Thursday?

It can happen after 5 years, 6 years or 11 years. There is not standard gap, due to leap years causing days to be skipped. If you take over the last 50 years the 25th of March was on a Thursday in 1965, 1971, 1976, 1982, 1993, 1999, 2004 and 2010. So the pattern in year gaps is 6, 5, 6, 11 and then the pattern repeats. After 2010 there is an 11 year gap, so the next Thursday the 25th of March will be in 2021.