Gruber's gang in Die Hard needed the reduced security and number of employees in the building for a nighttime Christmas party for them to get into the building more easily, but they also needed enough employees at the party to create a hostage situation that would get the FBI out so they could turn off the power, letting them drill into the vault.
You take Xmas out and they don't have their opportunity to make their heist.
And McClane is in that building specifically because it's Christmas.
No Christmas, no Die Hard.
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"If you wanna grow your business you need to exploit more." ~Austin_Era_II "Out of those two? Racist for me... easily." ~Vicious_Dios