P.S.
For supplies, I start as soon as they show up at the store.
For clothes, I buy as needed. Most of the stores have fall clothes by Aug, but here in Houston, it is still 90 degrees by the time school starts. If I buy fall clothes for my kid before the school year, he'll have outgrown them by the time it gets cold enough to actually wear them.
For actual lessons, I give my kid mini classes every day even in the summer. Since he's going into K we've been practicing writing letters, numbers, his name, reading, sight words, math, science, geography - even learning Spanish. I even make him to dot to dots still so he can keep his hand muscles strong.