work contract question

Hi there
I work here with a permanent contract. The exact hours change every year and currently I am contracted to work for example 5 hours on a Friday. Last week I was told I was not needed this Friday, I was to take the day off, I would make up the 5 hours another day to cover inevitable staff sickness, thereby saving the employer the cost of bringing in supply staff!

Anyone know where to look in Belgian employment law to know whether my employer can just decide to give me a day off I haven't asked for and oblige me to work another day I haven't asked to work either. Obviously I want to stay on good terms with my boss, but I suspect what has been imposed on me is not allowed at all and I should be able to say NO I will not work those hours on another day.
Thanks.