Irregular periods and ovulation

Does anyone else experience irregular periods? Usually it's the same day every month for approximately two-three months then the fourth month it will be two-three weeks late and follow this pattern. My cycle averages 34 days (past 3 months). I show that if I get my period on June 10th, I will be ovulating the last week of June. I have ovulation tests to check. But how do you calculate your ovulation with irregular periods?