Short answer. Religious grounds are the one preference the Labor Code tells employers to respect. Scheduling the weekly rest day is otherwise the employer's prerogative, but the article expressly directs that an employee's preference be respected when it is based on religion.
What the law says
However, the employer shall respect the preference of employees as to their weekly rest day when such preference is based on religious grounds.
Labor Code, Article 91 — Right To Weekly Rest Day. Read the full provision →
An exception written into a management prerogative
The article gives the employer the power to determine and schedule the weekly rest day, and then carves this out: however, the employer shall respect the preference of employees as to their weekly rest day when such preference is based on religious grounds. The structure tells you how to read it. Everything else about rest-day scheduling is management's to decide, subject to any collective agreement and the department's rules. Religion is the single ground on which the Code hands the choice back to the worker, and it does so in the language of duty — shall respect.
What the exception covers, and what it does not
The preference protected is one about which day is the rest day. It is not a general right to be excused from work whenever an observance falls, nor does the article address prayer breaks, dress, or leave for religious occasions during the working week. Those questions are answered elsewhere. Read narrowly and correctly, this clause does one thing well: it stops a roster from being drawn in a way that makes weekly worship impossible for those whose faith fixes the day.
Why the ground is limited to religion
The obvious question is why family, study or a second job do not get the same treatment, and the answer is in what the clause is protecting. A preference based on religion is not a preference for a more convenient day; the day itself is the observance, and no other day substitutes for it. That is a different kind of claim from wanting Sundays because everyone else is off, which is why the Code singles it out rather than opening the schedule to preferences generally.
Raise it in writing, and early
Put the request in writing when you are first rostered rather than after a conflict, state the religious basis plainly, and keep a copy along with any reply. If the request is refused, the reason given is the thing to preserve. Where a collective bargaining agreement covers rest-day scheduling, read it alongside this article, since it may already deal with the point. If the refusal persists, bring the written request, the response and your roster to a lawyer — those three documents are the whole of it.