When I was working, I tested the fire alarm every Friday morning at 10:00am. We had many members of staff who, because of the role that had, were not in the office every Friday morning.
So I arranged a Saturday test day for some people, so that they would recognise the sound of the alarm, if it were a real event.
So, Saturday the 1st April 2017. I 'invited' particular members of staff who I reckoned would not be very happy about coming into the office on a weekend, just to hear the fire alarm!
I sent out a blanket @mail and then I hid!
A few recognised the @mail as a prank, the date is an obvious giveaway.
But four of them were not amused. 1 in particular, Kevin, came to see me and he was fuming

He had his fists clenched tightly by his side, and tried to keep cool as he told that he had to go shopping on Saturday, with his wife, the dog wasn't well, the kids would need a babysitter.
I said to him "Kev, have you checked the date?" "Do you want me to pick another date?"
He couldn't see the connection. It was a guy who sat near to me that rescued the situation by explaining to Kevin the event was on April fools day!