It's considered bad etiquette to plan/host your own shower... has someone offered to give you one? A shower is generally a gift, if someone ill offer it will probably be sometime between now and 25weeks or so. Most showers are given 28-32 weeks (at least in our circle they are).
I started envisioning my ideal shower around 15weeks and I had absolutely already registered... but I wasn't "planning" anything just day dreaming, no one had offered yet. That happened about 19wks, I remember because it was the day before our anatomy u/s.