Tax planning is more than just filing your returns. It involves seeking and adopting techniques that can help reduce tax owed.
With that, October to Early November is the best time of year to start tax planning. NOT December.
It’s before the holiday madness AND before the end of year, so that you can contribute to retirement accounts or make any possibly tax beneficial changes to reduce your tax liability.
Once it’s January 1st of the new year, it’s too late to make any of those contributions or changes.
It also allows your bookkeeper, CPA, financial advisor, tax preparer, etc, adequate time to go through your financials to help you with these decisions before it’s too late.