Setting Perdioicity to N-Week, N-Month

I am trying to create charts based on periodicity of N-Week, N-month
(N=2,3,4,5...)

When I use an equivalent of n-days ( eg 10 days to indicate biweekly), bars do not end on Fridays for all symbols as I cannot determine how to define the start of week.

I have same challenge trying to define an N-week periodicity in the backtester.

I have read the guide at http://www.amibroker.com/guide/h_charting.html and
searched the forum on periodicity however most of them refer to N-minute/N-tick.

Apologies in advance if this a simplistic question I am still learning - All assistance is gratefully accepted.

Sorry, but as far as time-based intervals are considered, only N-second, N-minute, N-hour, and N-day, weekly, monthly, quarterly and yearly intervals are supported. You can use 10-day interval but it will start from first available data bar (not necessarily monday)