Day 84, 12:00, for 12 hours: 72 zombies at ×18 health and ×18 attack.
Part of Endless tier 17, day 4 of 5.
When it ends:
Scrap Metal
Broken Glass
All-Purpose Strengthener
Fine Sheet Metal
Instant Spicy Hotpot
Inspirational Story (Endless rewards, tiers 15-19)