Dr I am on ttc now, I want to buy a ovulation kit. Which day can I use it?? My periods are 29 day cycle, 3 days length. Last period was on july 30th

Usually done few day. The most likely time for ovulation is 14 days before your next period. In this case that would be day 15 of your cycle. If your period started on July 30, that would be Aug. 13 and it would be advisable to start testing on Aug. 11 and test daily until you ovulate or Aug 15 or16.