After each event, upload your attendee list and use a property (like “Last Event Date”) to stamp the date of that event. Then, build a workflow that triggers off that date, using delay steps to space out the emails:
Day 1: Send thank-you email
Day 30: Add an if/then branch to check if they’ve submitted a case → if not, send free case offer
Day 90: Check again if they’ve submitted → if not, send reminder
@GChen0 just a friendly warning here, that workflow, as pictured might not do what you described as a requirement. The first branch is checking whether a contact arrives there before April 30th – if you turn on the workflow later, all contacts go down the right path, a dead end. There is no need for this step 1 and 2. The workflow can, as I described, start with step 3 immediately, assuming that you turn on the workflow when you want it turned on.