An accounting practice already has a marketing calendar. It is the statutory one.
Practices that try to bolt a marketing programme onto a compliance business usually fail, because the year has no room in it. Every worthwhile month is already booked against filings, and whatever campaign was planned for the fourth quarter is abandoned in favour of the work that has a statutory date attached. The way out is to stop treating client communication as a separate activity. The compliance calendar already generates a legitimate reason to contact every client several times a year, and doing that well produces more growth than any campaign a small practice could run.
Reframed this way, the routine is concrete. Ask for documents earlier and more specifically. Confirm when a filing is complete instead of going silent. Raise the engagement renewal before the year starts. Notice what the compliance work reveals and act on it. Record who sends you clients. That is the whole programme, and it maps onto software cleanly because every part of it has a date and an owner.