Can they change my conditions?

[SquidgyMaltLoaf]

When I took my job a year ago it said there would be occasional evening and weekend work, which was fine. They now want me to work one evening a week most weeks a year, some weeks two evenings, and around ten full weekends per year. I really don't want to do this. Am I entitled to object or do I just have to put up with it?

Yes you can say you are not prepared to agree to that change in your working hours/terms and conditions.
But if you've only been there a year they could just give you notice and either just dismiss you, or dismiss you then immediately offer a job on the revised terms.

Attempting a compromise is probably sensible.