Undercovers is correct. If you're in a job where the workplace has to be covered over a bank holiday - a hospital, pub or some essential service, for instance, then you can expect to be asked to work some or all bank holidays. Depending on the terms of your contract, you may get paid extra for that, or you may be able to take the day at another time.
I work for a local authority and we're all given a set number of days' annual leave (currently 30 for me), but that includes 8 days that are classed as bank holiday leave, so my actual entitlement is 22, effectively. Our section doesn't have to open on bank holidays as it's not an essential service. However, colleagues in, say, parks or social services may be expected to work.