We don't wait for any certain time to have sweet potatoes or anything we grow,--When the urge comes up, just do it! lol!! I like Andy's racks!-- That is the only good way to store veggies! Keeps them from mashing the ones under them, PLUS you can spot bad ones before they damage others!
We needed to dig before voles/mice got real bad in them.--they did damage a few, but no great amount this year. ---Also wanted to get them before frost hit and ground temps dropped. If they been planted for 90-100 days, it's time to dig as they are done growing anyway.