If “refusal” to work on weekends is not related to a protected leave of absence, you would simply want to hold this individual accountable for any absence that occurs on a scheduled Saturday the same way you would any other scheduled shift. It is recommended that the expectation of weekend work is communicated to this employee verbally as well in writing. The consequences (negative impact to performance appraisal or disciplinary action) of not showing up for a scheduled work shift should be communicated to the employee as well.
Additionally, this information should be communicated to job candidates prior to employment in order to ensure they clearly understand the schedule requirements of the job.
The only exceptions would be related to protected leaves of absence and/or any reasonable religious accommodations, such as when an employee celebrates his or her Sabbath on a Saturday.