Going on maternity leave

I'm am 28 weeks pregnant and I work at a daycare there are 2 other ppl that I work with. I'm just wondering when would be a good time to go on maternity leave? I work with kids ages 8 months to 4 yrs and I do lift these kids a lot and I have a lot of back pain and my legs and feet hurt constant. I just want to go on maternity leave so I can get ready for baby and just relax till she gets here. So when is it best to do it?