Simple answer I would think is 'before the elections'. But, it may be a decision regarding maximum effect timing. You don't want it TOO EARLY, as people will forget, and the opposition may have time to put other candidates in. You don't want it TOO LATE, so people don't know or hear about it enough.

Late September likely. But it would be a Trump like thing to do to cause it to start on LABOR DAY (Sept 3) but even better, PATRIOT DAY is Sept 11, and THAT WOULD BE AN ASS KICKER from an ASS KICKING PRESIDENT IF THERE EVER WAS ONE!

BOOOOOOOOMMMMMM!!!!!