Event Tickets much like Premium Tickets are an in-game currency. They can be acquired each day of an event, and after each game as a random reward while an event is going on, but are not saved between events. They do not cause you to expend any energy to use, so you can play the event in the 15 minute intrums between energy replenishment. They are also used before any premium tickets are used.
Acquiring Event Tickets
Each day of an event lower teir of the event are given between 1-10 tickets (usually 10 at bronze and silver).
Random reward after each wave while an event is ongoing in the amount of 1-10, 1 and 2 being much more common than higher amounts.