Short answer. In six situations the Labor Code lists: emergencies from calamity, urgent work on machinery to avoid serious loss, abnormal pressure of work from special circumstances, preventing loss of perishable goods, work whose nature requires continuous operations, and other analogous circumstances determined by the labour department.
What the law says
In the event of abnormal pressure of work due to special circumstances, where the employer cannot ordinarily be expected to resort to other measures
Labor Code, Article 92 — Work On A Rest Day. Read the full provision →
A closed list with one deliberate opening
Five of the six grounds are specific: calamity emergencies to prevent loss of life or property or danger to public safety; urgent work on machinery, equipment or installations to avoid serious loss the employer would otherwise suffer; abnormal pressure of work; preventing loss or damage to perishable goods; and work whose nature requires continuous operations where stopping would cause irreparable injury or loss. The sixth is the opening — other circumstances analogous or similar to the foregoing. Note who decides what is analogous: the article says it is determined by the Secretary of Labor and Employment, not by the employer.
The busy-season ground has a condition attached
The ground employers use most is abnormal pressure of work, and it is the one whose qualifier is most often dropped. In full it reads: in the event of abnormal pressure of work due to special circumstances, where the employer cannot ordinarily be expected to resort to other measures. Three limits sit in that sentence. The pressure must be abnormal, not merely heavy. It must come from special circumstances. And other measures — additional staff, rescheduling, overtime on working days — must not have been ordinarily available. A predictable annual peak, planned for every year, tests badly against all three.
Serious and irreparable are not decorative words
The machinery ground requires serious loss, and the continuous-operations ground requires that stoppage may result in irreparable injury or loss. Loss that can be made good by working the following day is not irreparable, and inconvenience is not serious loss. Reading the grounds with their adjectives intact is what keeps the list from swallowing the rest day altogether, which is the direction it drifts in practice.
Rest-day work is premium-rated work
Whether or not your employer was entitled to call you in, the hours you worked on a rest day carry the premium the Code provides for. Those are separate questions and it is worth keeping them separate — an employee arguing that the call-in was unjustified sometimes forgets to claim the pay for the day actually worked. Keep a record of which days were scheduled as your rest day, which of them you were required to work, and the reason you were given at the time.