If that's with cash then I'd wait until January 30. You can at least earn ~29 days more bank interest on your cash.
If you're planning to transfer SA dollars to RA then it'll be best to do that within the 1st half of January, before the CPF Board starts closing Special Accounts. January 1 works, of course.
I don't think you can obtain tax relief with a cash top up to your RA. Your RA is already way above the 2025 Full Retirement Sum ($213,000). It seems doubtful that your RA has so much RA interest that you still have room below the FRS for tax relief purposes.
If you've got gobs of cash sitting in a 1% interest earning bank account, for example, then it'd probably be a good idea to use your cash first. OA earns 2.5% interest, and that obviously beats 1%.
But another option that might be even better is for a qualified family member (or family members) who happen to be under age 55 to transfer their OA dollars to your RA. Then you can return the favor (make cash top ups to their SAs) and/or even hand them cash. The reason is that they cannot withdraw OA dollars since they're under 55. But by doing something you already plan to do (add funds to your RA) they (and you) can effectively "liberate" their OA dollars (make them liquid). And that's not a bad thing, assuming they're going to do something reasonable like save and prudently invest those liberated dollars. Buying lottery tickets or tobacco would not be on the list of reasonable things to do.
Again, you shouldn't make a Voluntary Contribution to MA for tax relief on January 1 unless you're specifically trying to beat December's payroll cycle or a pending other deposit into your MA (such as an insurance claim that's still pending). If for example December's payroll cycle ordinarily gets credited to your CPF accounts on January 10 then make your Voluntary Contribution on January 8 (2 days in advance). That'll mean you'll earn 7 more days of bank interest on your cash, and that'd be a good thing. (You can send me half the extra interest you're now going to earn.

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