Two categories will perform on Saturday night, with the remaining two performing on Sunday night. One act will be eliminated on Saturday and one on Sunday most weeks (no sing off at all) and there may be weeks where 2 acts are eliminated each night (which will be one act from each category)
This format stays until the semi final (week 5)
All the details here:
http://www.itv.com/terms/votes-the-x...ing-terms-2017