Can i make up my missed fast on Saturday or Sunday?

or do i really have to start on Friday. I really want to make up all my missed fasts and do the 6 days of fasting during this month of shawal. I've been fasting for four days now since monday but due some reasons i cannot fast on this Saturday. But i really like to continue fasting on Sunday. However, my intention is to make up my missed fasts and not doing the sunah ones.

Please advice.

Thank you and best regards,