It could be a matter of convenience. School holidays makes it easier for school children from former relationships or marriages to attend the wedding.
Then there are sentimental reasons. The date chosen might be the one when the parents of one of the partners exchanged their own wedding vows.
Some dates are those associated with something significant like St. Valentine's Day, last day of the year, first day of the new year, or easily remembered days such as the 11th of the 11th of the 2011.
In different cultures there are also lucky days on which to get married, which could be any day of the week.
Recently I had a couple who chose to be married on a Monday.
No, it's not their lucky day. It just happens that he was born on 11th of February, she was born on the 12th of February, 14th is St. Valentine's Day, so they chose the 13th to keep it symmetrical.
Each year they will have four consecutive days to celebrate both their personal days and their together days.
