did they ever mention when they were married? I know they showed them getting married in a 2-part flashback episode but did they ever mention the date of it?

I think they got married in 1969. In the final episode that Mike and Gloria were in they renewed their wedding vows along with Archie and Edith. Gloria mentioned that they were going to do it next year when they had their 10th anniversary and Archie and Edith had their 30th. They renewed early since they were moving to California. This was 1978.