Toreador

I'm joining Jake and Pete for a track day next monday, but my rear tires are worn and need replacing soon. I still have some tread on them, but not muck. Should I get new ones now or wait until after the track?
New tires are going to need breaking in, and I don't think I have time to do that before monday.

TrackJunke

Keep the old tires. With some brand new street tires, with full tread, you can sometimes get tread skrim which will create excessive heat in the tire and can sometimes ruin them.

Palting

I'd go with the old tires, so long as they are not down beyond the wear bars. New tires need to have the release compounds scrubbed off with many miles of driving otherwise they are slippery for the track. Tall treads will also be more squirelly and can chunk easier.

Just keep a close eye on pressures and tread, and maybe slow wa-ay down if it starts to rain.