Create a player, then go to "Edit Player" at the roster screen. Press Up or Down to choose the player you want to edit. Press Right to get to the speed category. Press A, then press A again.
Set the system date to the third Thursday in November to hear Thanksgiving commentary.
Set the system date to December 25 to hear Christmas commentary.
Set the system date to January 1 to hear New Year's commentary.
When your player is wide open, press L or R before running into the endzone and your player will do a dance.