tl;dr: August, once I know what kids I have in my band, and what their strengths/weaknesses are.
Long version: Your music selection should always be about 6 months out. You should know your marching show in March, and your contest music in August/September.
Don't set anything in stone - but if you don't have an idea for a march and three or four possibilities for your list tunes by the end of the first six weeks, you'll feel perpetually behind for the entire "season" of that music. Unless you have a very late UIL, you'll struggle to catch up until late.
When I worked in schools that had UIL before Spring Break, I had my march picked out and we performed it on the Winter concert every year. Learning three pieces to a UIL standard, during Solo & Ensemble time is very, very difficult.
By the end of SW1, you should know where your grade problems (and teacher problems) lie, where your strong sections and weak sections are, and can program accordingly.
Don't force a piece in just because you want it if your band can't play it, or your don't have a strong section that it calls for. Pick things that suit your strengths. Don't play Holst 2 if you don't have a euphonium soloist. Don't play Puzsta if you don't have a really ripping clarinet section. Don't play Nottingham Castle if your flutes can't tune. Don't play Flight of Eagles if your woodwinds can't play independent eighth note parts for extended periods.
TYB is good on offering advice for Grade 3-5 pieces. The Grade 1-2 lists are getting rather thin at this point comparatively (fewer pieces, more "dead weight" pieces that no one plays, more "everyone does these" pieces) so there's only about 15-20 pieces on the list that people recommend. Ask here, but be willing to use them as a starting point. There are gems on the Gd 1 and 2 lists that never get played because they're old or few people know them.
Use BrynnPark also to help yourself find pieces. Find music that other bands have played successfully at contest. Since it was pulled off the list, I don't need to say "Don't play Ticheli's Portrait of a Second Division" but there are a BUNCH of trap pieces on the list that look hard and are VERY VERY difficult to play well (Crusade by Gassi, anyone?) because of tuning or odd "lays" in woodwind parts.
Once you pick your music, reverse-plan out your entire Spring semester from UIL back. By 2 weeks before UIL, we should be working (X). By 3 weeks, we should be able to play (X). By a month out, we should be able to make it through the entire piece of our difficult piece. Plan backwards and plan it out. That'll tell you when you need to start working the music, too - if you pick a piece that needs 12 weeks to prepare properly, and 12 weeks puts you starting it in November, then start it in November!
Finally, remember this: Add at least a week to your estimated prep time in the spring semester due to testing, absences, county fairs and shows, softball/basketball/baseball/soccer/tennis/track meets/other sports, spring fever, and everything else. If you have said 12 week piece, it'll really take 13. If you're in a small school, double that time. You will not have a single rehearsal from TMEA until UIL where you have 100% of your kids unless you are VERY lucky.
I always start very early with a few things in mind. I am constantly thinking about it it throughout the fall, researching more pieces, thinking about how to teach it, how long would it take...etc. I have had a few people tell me I shouldn't worry about it until later. But I feel that selecting music is the most important thing to get right. I feel its important to exhaust every option and researching a ton in order to make the best and most informed decision.
What do you think?