Calculation Rule -Planning Calendar

Dear all ,

Can any body explains the concept of calculation Rule in planning calendar ,

If i choose the Calculation rule as weekly the screen appears like bleow

Period:weekly

periods beging with following work days , here i can enter up to 6 days , How this period start date is effective ...

For Example : If i enter Thursday as in the first row , it means the always the period starts on thursday ...

If i enter Wednesday , tuesday in the next row what it exactly means? How the system take this periods during the planning run .

EX

1st Row : 4 - - - >Thursday

2 nd Row 3- - - - > Wednesday

3 rd row 2 - - - - > Tuesday ...

Regards ,

Kumar