Vacs are spring and fall, my vet sends advance reminders. If this service (vet reminders) wasn't available to me it is unlikely I would forget, but I would use the calendar in MS Outlook to remind me to purchase vacs at the appropriate time.
I use the Outlook calendar to remind me of all major expenses, like home/auto insurance premiums, vehicle registrations, and property tax liablility. Else time slips away and and when a get an "unexpected" bill in the mail I freak out. I don't have experience with other calendaring programs but I'm sure there are many. Advance warning of major expenses enables me to freak out earlier and savor the experience longer
