March 1st
It says: "I understand that beginning on March 1 and thereafter, when considering multiple offers of acceptance, I should hold only one position by accepting the offer of one dental school and declining other offers. After this time, if I hold offer(s) at more than one dental school, my status will be shared with all applicable schools and may result in offers being changed or revoked."
About the 14 day notice. The link that artist2022 so kindly provided says that if you are holding multiple seats, schools can give you at minimum 15 days notice if they choose to withdraw you. But how long depends on the school. "Dental schools with candidates holding multiple positions on March 1 of the year of admission will give such candidates a minimum fifteen-day notice if they choose to withdraw the candidate from the entering class."