On Thanksgiving day itself, you're likely to find a seat. Others will not allow you to fly standby on a flight if all the seats in your fare class have sold out, even if there are empty seats available. If the agent at the check-in counter gives you trouble, go directly to the gate and see what happens. If you want to try flying standby on one airline using another airline's ticket, bypass the check-in agent and go directly to the gate. Crew who must reach their destination for work purposes have priority over all non-revenue passengers.
Active employees have priority over retired employees and dependents of an employee.