Best vs. Worse time? Planning a pregnancy to avoid being pregnant in the third trimester between july-september is a common occurence here in texas where it routinely exceeds 100 degrees in the summer. However, from a medical point of view there isn't a "best" or "worse" time except within 3 months of a pregnancy loss to allow the body time to recuperate. If you are already pregnant, work until complications or labor.
Postpartum. The first 6-12 weeks after a delivery are the most opportune time to take maternity leave; that's when your baby (ies) need (s) you most and when you have the most need for recovery.