Generating shift pattern in SQL

I don't know even how to start addressing this issue, any assistance will be appreciated:

My goal is to generate a table (dynamically) that return dates that are working shift.

Settings:

• I have a pattern as follow 7 days on duty 7 days off duty and it goes on...


• I can determine the start date that the shift start


• I can determine the pattern days (7 days - meaning 7 days on 7 days off)


• i can determine the end date for calculating the patern


• I want to calculate and create the following table

For example: Pattern days: 7 Start date: 01/01/2015 Pattern end date: 12/31/2015

ID         StartshiftDate        EndShiftDate     OnDuty
-------------------------------------------------------------
1           01/01/2015           01/07/2015       On Duty
2           01/08/2015           01/14/2015       Off Duty
3           01/15/2015           01/21/2015       On Duty

I know that i need to create CTE that start from the start date, i need to add 7 days for each date. but i don't know how to determine if the range of dates is on duty or off duty.

And how i create the the loop for creating the row till the pattern end date?

Any help will be appreciated