明日は休みだったのに出勤になってしまいました。って英語でなんて言うの？

1. You explain that you were looking forward to a day off, but someone (presumably your boss) has insisted that you work instead.
   
   2. Sarcasm is an intrinsic part of the British psyche - you actually feel quite angry about your day off being cancelled. Actually in the UK, employment law respects employees' working hours and anyone may easily refuse to do hours above their basic contracted hours.
   3. This is an informal and spontaneous reaction to having a day off cancelled.

If you were meant to do something then you can also say you were 'supposed' to do it if you are describing soing to work then you can simply say 'I'm working' as well as 'I have to go to work'

"I was supposed to have the day off from work tomorrow, but now they want me to come in." Supposed means "going to" in this sentence. Have a day off means not have to work. Come in means to go to work in this sentence.

"It was supposed to be my day off tomorrow but not anymore! "

This a very straight forward expression. Most people will not be happy to lose a day off so in this expression even though you are being formal you are also showing some emotion by adding "but not anymore" by adding this you are letting the person you are talking to know that you no longer have a day off!

"the day off" is a term used mainly describing a work situation, to not go into work or other activity.
using present tense "I have" opposed to the past tense of "I had"

These three answers are the same as saying "I was supposed to have the day off tomorrow, but now I will have to go to work."
You can use any of these answers when for example, talking to a friend who you may have made plans with and you need to cancel those plans because you have work.

Annual leave is a number of days per year which employees can use to have time off work on specific dates.