January 1st is just a Tuesday with better marketing. Start whenever you decide to.

Waiting for a starting line is a way of postponing while feeling organised about it. The calendar has no opinion on your habits, and a date carries weight only because everyone agreed to treat it as a threshold. Meanwhile the change you are saving up could have had weeks of practice by now.

Pick the thing you have been holding for a cleaner moment and do a small version of it today. Five minutes counts. Starting on an ordinary day also removes the pressure to be perfect at it, which is usually what ends the January version.