How do you know your turnip is ready ?

[Medina]

I can't remember which day i planted my red turnip :S Do they mature or change so that I can tell it is ready to be picked ? and will it die if i leave it in the ground too long ? Also if i accidentally dig up a turnip and rebury will that damage it and or restart the 6 days before i can sell thing ?

Thank you in advance for all your help