Hubby works in an office/scientific environment and they all have shared calendars that link to a central source. (FYI UK based so normally we have a Bank holiday on the last monday of May. This year we have a special double Bank Holiday the following week for the Queens Jubilee instead).
So yesterday he's yammering on about gardening and says "oh, we will take the garden waste to the dump on Monday so that we can do X first..". I look at him strangely as I know he won't be home in time from work.
Turns out his work calendar shows both the coming Monday and the Jubilee days as bank holidays. If I hadn't had this conversation with him he might well have not realised it wasn't! The thing we expect is that they have a repeating entry to put in a 'last monday of X' bank holiday every year and someone added the Jubilee dates manually not realising.
(It does occur to me to wonder how many other people won't realise/won't turn up though. If it was me I'd be tempted to joke around with the bosses - if they were good natured - about it in a "we thought you were giving us an extra day" way!