Change in employee's rest day must be thru WRITTEN notice 1 week before effective date

Where the weekly rest is given to all employees simultaneously, the employer should make known such rest period by means of a written notice posted conspicuously in the workplace at least one (1) week before it becomes effective.

Where the rest period is not granted to all employees simultaneously and collectively, the employer shall make known to the employees their respective schedules of weekly rest day through written notices posted conspicuously in the workplace at least one (1) week before they become effective.
An express waiver of compensation for work on rest days and holidays provided in an employment contract which fixes annual compensation of the employees is not valid and does not operate to bar claims for extra compensation therefor.

Rest day cannot be offset by regular workdays.