What day is today?

Monday  --  week-one

Tuesday  --  week-two

Wednesday  --  week-three

Thursday  --  week-four

Friday  --  week-five

Saturday  --  week-six

Sunday  --  week-seven   （week-day）

week-eight ,week-nine

What's the usage?
If there are 8 or 20 days in a week, it will be useful
And it's easy to form words