Most calendars in the US have the week starting on Sunday and ending with Saturday. Calendars in S. America and in many parts of Europe have it beginnng with Monday and ending with Sunday.
Basically there are no rules, except what is 'tradition'. I like the idea of having a 'week-end', so I put Sunday as the last day.
There is some interesting calendar history as to which day goes where, and after reading it all I still decided to put Sunday as the last day of the week.
Hopefully it doesn't mess too many people up.