"Yes, I am busy on the weekends."
According to another comment I read に isn't used for regular occurences like "every weekend" (or in this case, "weekends"). I also believe は is optionally used for emphasising the time; it accepted my translation of はい、週まついそがしいです。, which you may notice does not include the は.
Hmm… possibly the non-Japanese comma…? That'd be a bit weird if so though… Maybe they didn't expect anyone to switch to standard commas mid-sentence?
Or if it was a type-what-you-hear lesson, I've heard they must be answered in exactly one way, so Julia might be right about it possibly wanting different kana (unless that's been changed; I don't actually remember how old those comments about "one answer only" were).