First day of Indoc. If you are in the Sept 21st class and you get the MD88, you can bid off the MD88 on the first Advanced entitlement on or after Sept 21, 2016.
Subsequently if you are awarded a different aircraft on an AE then there is a 24 month aircraft lock. However its not a seat lock, its an aircraft lock. Big difference. There are a few clauses to the aircraft lock, bidding for a new category in a domicile (ie they open up a 777 base in LAX, you as a new hire MD88 FO can bid off the 88 for the 777 in LAX if your seniority can hold it) and another is by upgrading (which in the first year haha).