Is day 21 of a 29 day cycle a safe day?

If regular cycles: If your cycles are regularly 29 days, then you should have ovulated on day 15, meaning that you should be safe following day 20. So yes, day 21 is a safe day as long as you have regular periods at 29 days. Hope that helps. Condoms and hormonal contraception are still more effective methods than the calendar method. Please consider them if not trying to avoid pregnancy.