Yes! All levels of Six Flags Memberships allow you to visit both Fright Fest and Holiday in the Park an unlimited number of times.
Some levels of Memberships come with extra benefits at Fright Fest and Holiday in the Park. For example, With a Diamond Membership you can visit one haunted house at no additional charge. With a Diamond Elite Membership you can visit ALL of the haunted houses every time you visit at no additional charge. At Holiday in the park, you Diamond and Diamond Elite Members get FREE s’mores.