Does cancelling an agreement with a party planner six days before the event entail legitimate compensation, especially since the planner claimed two other events were cancelled because of me, noting that the agreement was verbal and without prior confirmation?

If the party organizer incurs damages due to your retraction of the promise, you are obligated to compensate them for the actual damages incurred. This is in accordance with the resolution of the Islamic Fiqh Academy, which religiously obligates the promisor to fulfill the promise or compensate for damages resulting from its unfulfilled nature without a valid excuse. However, if the organizer could have avoided the damage and did not, then you are not obligated to anything.