#1
All
Actually this isn't a question only for big four but what made me think of this question has to do with the Big Four. I noticed Megadeth is going on before Slayer. I was suprised because Megadeth has sold more albums and enjoyed more success.

I have always been curious how or who determines the band lineups in festivals whether it be the big 4 or some other festival. Can anyone offer some insight?

TX
woem
#2
randomly, nobody in charge would give a **** who goes first or last.
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#4
Usually it is done by "size" - seniority, popularity, etc. There's always exceptions. And some people may argue Slayer is "bigger" than Megadeth.
#5
In other festivals, it will have lots to do with when they are available during that day. For example, if they want Behemoth to play, but they need to catch a plane at 9pm to get to the next festival or whatever, then obviously they need to play before they have to go to the other place.