Ovulation kits question - cycle days

Okay, so I was on birth control (the pill microgestin) for the last 10 years, I have been off it since mid-march. While on it my period was like clockwork- 21 days followed by 6-7 day period. My first two cycles off the pill went the same. However, the last few have come a few days later each time. So my most recent period, which just ended today, was 6 days "late". So I'm confused about my cycle days count and when I should start my ovulation test kit since it says you are supposed to start on a specific cycle day. Can anyone offer any advice on this? I'm 35 yo and I have two boys from a previous relationship, that weren't even really planned, but now my boyfriend and I have been trying for 5 months without luck.

Thanks in advance!