31st

Receive a memo on 31st March from an extra-terrestrial:

Dear Physician,

You are the ward physician next month; MOs A and B will be under your supervision. Please note that MO-B will be assigned to other duties on the following dates and will not be under you:

1-9, 10-19, 20-23 and 24-30 of April

Sincerely, etc.

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Dear Governor of the Universe,

Does your highness mean that there is 31st of April on the planet that you live ?

Your humble servant on Earth

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By the way, you may find a very similar comedy in the last Chapter of Yes, Minister. Since I don't want to be thought of impinging the copyright of Antony Jay and Jonathan Lynn, I must declare the above story is genuine.