I've chosen the winners of the Plain Language writing contest. The following entry from @Jamaal Valentine was the acceptable entry with the best readability score:

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If events beyond the government's reasonable control interrupt normal receipt of offers, and prevent change of the exact due date or time, offers are due at the same time on the first work day that normal government operations return.

The most humorous entry came from @apsofacto:

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Life.  Wow, man, Life. 

It's really strange you know?  We make these plans to receive your proposals and then Life happens and we can't.  We can't even issue an amendment to our solicitation!   Could be plague, could be invasion, could be we just totally flaked out that day. 

But chill.  We'll take it when we get back.  We won't trip about Life.  Cuz. Life.  Trips. Us. Man.  All of us. 

Can you pass that stuff over here?

 

There were some others I really liked that didn't win. If we slightly change @bentley78's entry, we get:

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If we can’t receive your proposal on the due date and we forget to extend the due date, you can assume that it’s been extended until the next day we can receive it.

Thank you all for participating!