1) "Mary was sick last Monday"
2) "Mary was sick on Monday"
3) "Mary was sick Monday last"
In the UK these have slight differences depending on where you live.
1) means that Mary was sick, not on the Monday of the current week, but the Monday of the previous week.
2) means that Mary was sick on the Monday of the current week.
3) means the same as 1) although I don't hear very many people phrase it like that. 1) & 2) are most common. Note that we wouldn't usually say "on" monday last - just 'monday last'.