Is there any difference in transferring in the month of Dec 2020 and January 2021 for fund from CPF OA to SA?

As the interest for CPF is calculated monthly, the difference will be at least 1.5% p.a. for a month. I mentioned at least, because if your total CPF is lower than 60K while your OA has more than 20K, you are not fully utilising the bonus 1% interest which you would have gotten if you transfer from OA to SA.

Transfer from OA to SA offers no tax relief, so no difference in that aspect.

Interest wise, you transfer in December means that amount will attract the 4% pa interest (instead of 2.5% pa) next year. Transfer in Jan means one less mth of that interest. But I would think this will not affect anything.

There is one thing that would be affected - if your SA account has reached FRS balance (181k for this year) or not. For the OA to SA transfer, you can only transfer to top up the SA until 181k for 2020 or 186k for 2021.