Week 39 is the best deal on the current FW charts for both RIV and Poly because the FW charts were developed in 2023 when Week 39 was fully in September but this only happens around every 7 years with the rest at least partial October dates with much higher point costs.
There are some other weeks with some savings some years but nowhere near the max 20 points per week in savings week 39 has for 5 years through the life of the contract.
To spot other options look for weeks fall at the end of a low point period with a higher point transition after then look at the 2023 calendar for when transition weeks mostly fell in the lower point period. These weeks are the end of January, mid February and Mid May. Other possibilities are weeks with potential holidays (Easter and Thanksgiving) but didn't have a holiday in 2023.