Can u ovulate a day or so diff every mth?-my cycle is 26-28 days, started opk on day 13 ,14,15 i think i either missed it by a day possibly,

Yes: Not every period is exactly the same. It is definitely possible to ovulate on different days in different months.

Yes: Every month you may ovulate on a different day depending on your hormones and cycle length. 1 to 4 days variation is common. In a 26 day cycle, you are ovulating approximately on Day 12 plus or minus 2 days and in a 28 day cycle, you are ovulating on Day 14 pus or minus 2 days. It is best to be sexually active at least every other day during that time frame so you do not miss it. Good Luck!