I arrive in Paris (CDG) on Monday 30 Dec 2019 and already have a Navigo pass (purchased and used 3 years ago). I will be staying for 6 weeks. As I understand, even though I can pay and load one month's value on to my Navigo card, it is not effective until 1 January 2020 - which means I cannot use this card on 30 Dec to go from CDG to Gare du Nord. So I will have to load one week so I can use it immediately or pay for individual tickets. Is this correct? Many thanks
If you plan on using your Navigo card to travel a lot on Dec. 30 / 31, then you could buy & load Navigo Day pass onto your Navigo card for one or both of those days.
The cost of 1-5 zone Navigo Day pass is 17.80€ (as of 2019), so perhaps it's more cost effective to use a single RER ticket for RER B train CDG to Paris on Dec. 30 and have a 1-2 zone Navigo Day Pass (7.50€) for Dec 31 and a Navigo month pass for January. You can purchase all of this at the same time in a single transaction when you arrive at CDG. (You can specify the active day of your Navigo Day Pass, up to six days in future from the day of purchase.)