if i understood your use-case correctly – You want trigger Mail-Notification to the end-user if his/her task end-date is going expire in next day ?
If so, i suggest this way-
Step 1: We can add one boolean attribute flag(Yes/No) in same entity(in your example it is Event)
Step 2: Create one microflow that polls this entity and do retrieve action on Event entity using XPATH (you can keep one day before Date logic here).
Step 3: Then for the returned records you can keep logic to trigger mail-notification(Sub microflow).
Step 4: Once you mail-notification has been sent, you can set the boolean flag to “Yes” for that records, so that in your next run of the job(Step 2), you can easily filter records based on this flag(otherwise, it will apply date logic for all records and if you have huge records, then this will work in optimized way.