If you buy a Navigo month long pass on January 20th, will the pass be good from then until the end of February or will it only be good from Feb. 1 to the end of February? In that case would you have to buy weekly or day passes to cover Jan. 20- Jan. 31? Hope that makes sense.
A Navigo month pass purchased on January 20 will be good starting from the first day of the next month and not valid for Jan 20 - 31.
So a Navigo Month pass purchased on January 20 will not start working until Feb 1 and will expire on Feb 27 or 28 in leap years.
You would have to buy Navigo week passes for Jan 20 - 27 and Jan 27 - Jan 31 and then your month pass would start validity on Feb 1 - end of February.