。
I'm free next Sunday.
Literal
Next-week [possessive-の] Sunday [if-なら] open-is.
~なら is the topic-based conditional 'as for X, if we're talking about X.' Used when the speaker responds with information keyed to what was just mentioned. 空いてる is the casual い-dropped form of 空いている ('is open / free / vacant'). The speaker's day is 'open.'