I voted YANBU - and then was amazed at the poll results so far!
However, having read the posts, posters make valid points, that I hadn't considered.
In my workplace (university), we are often expected to attend / participate in events outside of our core roles or core hours. It's often part of the 'wider good', like a community event, or to support some higher University initiative or so on. I'm at a level where I completely expect to do this, but it's really an expectation of all staff. However, staff are not forced or compelled in any other way - we genuinely, in the area I work in, have a good working environment (in that respect at least!) where colleagues are quite willing to do this.
I also worked in the private sector, in a consultancy; there weren't analogous events there but it wouldn't even have been considered to 'ask' - there were many times where we were expected to work beyond set hours or travel to site meetings at inopportune times. Yes, in theory, a 'no' might be possible. In reality, if you wanted to proceed and do well, you would do it - and again, most people broadly wanted to, knowing it would be rewarded in some other way, if not directly, and to give their clients the best service.
However, the comments do make some sense - I agree, it would be more sensible to work out your staffing requirements, roster staff and reward them appropriately - e.g. overtime or additional time off (not just 1 day in lieu).
I can't get over the posters saying they would tell OP to 'fuck off'. (Whether literally or less directly). If I'm not happy with something in work, I do my best to be an adult and say why, politely and see if there's an accommodation that can be made. I'm pretty certain in this case I would see the benefit to the firm and want to support it - although I do agree there were probably better ways of communicating the message to staff.