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My husband began in 2018 at our village gas station to help them out, as they were short staffed (Mon and Tues). He's 66 now and works just Mondays (past 2 years). They say he has to work weekends now too. This is a first nation reserve and weekends are crazy busy (cheap gas). It's too much for him. Can he refuse as has no contract? He can't physically do it.
Assuming his contract does not permit the employer to change his days of work, your husband is permitted to refuse to work on weekends given that this is not part of the terms of his employment. He could also get a medical note supporting this restriction.