The Winterfest and Summerfest slot tourneys USED to be cattle call, run 'em through the mill type, though not as bad as that sounds. Not invitationals, and not much in the way of perks like you get with their invitationals. Anyone and everyone was eligible. Staff used to roam the casino giving away the entry blanks to players.
What you got was 10 minutes or so on a machine in a group of machines roped off from the rest of the casino, 20 players or so at a time. If you "won" you got to play again in another round, and so on. I never did them, but did watch the action at time or two.
Things have changed and this could be outdated information.
Harrah's "real" invitational tourneys used to be excellent with lots of perks, especially food. Haven't done one in many years.