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Days are single-player missions. As you progress through the days the zombies will become stronger, making it so they can withstand multiple hits from the IonCannon (Ion Cannon), even if it is maxed out. Such instances start from Day 102 and up. Not forgetting also their damage increases as well. The duration of the days will also increase as you progress. Daily missions are not exactly the same as normal days. The tutorial stage is NOT considered a day.