Can i take a muscle relaxer if i've missed a day of my ssri?

Many muscle relaxers and SSRI can react together to rarely cause serotonin syndrome. This is rare but it does happen and you cannot predict when. It would be best to continue your SSRI because you are taking it for a reason and use other methods to relax your muscles like a hot bath/shower, heating pad, light exercise, massage, acupressure, yoga or rest.