many people think of tuesday as the third day of the week but according to international standard ISO, it is the second day of the week!
So according to the International standard ISO, they week starts on Monday?
yes. also the name tuesday derives from the old english "tiwesdæg" and that means "tiw's day"
Do all the names of the days of the week derive from old english?
im not sure about other days  but youre probably wondering what tiw is, tiws day was the day of tiw or tyr. tiw is the god of single combat, victory and heoric glory in norse mythology.
Was tyr also a god in norse mythology?
yes tyr is tiw. some things just have multiple names if that makes sense. but tuesday has other originations according to other cultures, in some slavic language tuesday originatted from old chuch slavonic word "въторъ" which means the second. it probably stands for second day of the week 
Is old chuch slavonic a language where old english rooted from?