A girl who is sometimes hot, depending on the time of the year. Often this includes chicks with very pale skin, who are only hot during the summer when they are tan; or slightly fat girls who are ugly when they show skin
"'Anne the Beast' is one of those people who's hot when she isn't all pale and disgusting"
"Yeah, she's seasonally hot!"
"I like her more when she doesn't show skin"
"You're right, she's seasonally hot"