I've had two just either side of new year's, both on balmy 45 degree days. Singlets and nappies is pretty much it, get sleeveless onesies with snaps at the crotch when they start taking their own nappies off.

DD#1 was in 0000 for several weeks, then 000 for several months.
DD#2 was in 0000 for several weeks, then 000 for 2 weeks (I was breastfeeding at the time, so those were an interesting 2 weeks), then 00 for the next few months.
DS looks like he's about to pop the seams of our last 0000 onesies (Target ones are quite roomy) and he's 5 weeks.