Effective July 1st, 2012, if you renew your validation sticker online through Service Ontario close to or on the date of expiration, you will have a 10 day grace period (beyond the expiration date) to receive and affix your new sticker. You must however keep the receipt for the renewal on you until you receive your new sticker.
http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_900628_e.htm#ys11p1s1
http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_900628_e.htm#ys11p1s1