Absolutely. You can cancel your subscription at any time, and you will still have access to all premium RSVPify features up until the conclusion of your paid subscription period.
In other words, if you subscribed to a monthly plan on January 1st and canceled your subscription on January second, you would still have paid access through February 1st.
Note that Single Event plans are a one-time fee for one year of access, and no cancellation is required. Your subscription will automatically expire 1-year after your upgrade, and no action is required on your part.