"do it until friday" ??
http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00016.htm
Díky za vysvětlení
"until" is wrong here. See the link in kacenka9's post.
Does "do pátku" include Friday?
Yes, it means "not later than", just like "by Friday".