How to tell if your cycle day is 28 or 32

I'm not sure because I was on birth control with a regular period that lasted 6-7 days and always started on either Wednesday or Thursday. Now I'm off trying to concieve , and my periods come when they want . And it usually lasts 5 days for example. I was on my period on May 25th 2016 that lasted about 6-7 days the 7th day is when I completely stopped bleeding  the 6th day is when I was extremely light almost spotting. Then I got my period June 17th and it hasn't came until July 19th . June 17th was on a Friday and July 19th is on a Tuesday. Please help me .