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,

Wassallam