If your cycle is 21 days or less, you should start testing on day 5 of your cycle. If your cycle is longer than 40 days and you are using either Clearblue Easy Ovulation Test or Clearblue Digital Ovulation Test, you should start testing 17 days before you expect your next period. If your cycle is longer than 40 days and you are using Clearblue Advanced Digital Ovulation Test, you should start testing 20 days before you expect your next period.