If my cycle is 28 days aprox. what day should i have a blood test to see if i'm ovulating?

Calendar: keep a calendar of your cycle for a few months, if you are regular, you always ovulate 14 days BEFORE your period starts. once you have a good idea when to predict your start date count backward 14 days and that should be the date you will ovulate. if you are a clockwork 28 days however, you can assume it is 14 days after the start of your next period