Puzzle ZYFP 

Workings

Hint
Try working backwards.

Answer
Saturday.

Reasoning
It's best to work backwards:

   "Sunday's tomorrow"

is Monday.

   "the day after Sunday's tomorrow"

is therefore Tuesday.

   "five days before the day after Sunday's tomorrow"

is therefore Thursday.

   "yesterday was five days before the day after Sunday's tomorrow"

yesterday was Thursday, making today Friday. Therefore tomorrow is Saturday.