How do I remind customers about a seasonal tune-up or service?

Three ways, depending on how far you want to go. Customers → At Risk lists who was active and has not paid you in ninety days — the people a spring or fall reminder is for. Ask Des to write to them ("email everyone I haven't seen since March about a tune-up") and it drafts one note per customer for you to approve before anything goes out; a campaign to the whole list is the AI Partner Email add-on. For customers who should be on a schedule rather than a reminder, a maintenance plan — the contract under Get Paid → Contracts for the money, and its visits under Jobs → Plans for the work — puts the visits on the calendar and bills them on its own, and the day-before appointment reminder goes out to each visit once you turn customer reminders on.