When to start baby shower planning?

I'm having my baby shower the last week of October when I'm 31 weeks along. I wanted to have it early enough that I can set up the room and exchange things if I need to. I also teach kindergarten and will be working as long as possible so I don't want too much stress at the end of my pregnancy. When should I start planning the shower? As in picking a day and talking to my mom and mother in law that are throwing it to think of ideas and places? I'm not sure. How far in advance people do this. Thanks!!