Learn German: Der Sonntag ist ihr heilig.

The standard phrasing is jemandem heilig sein (to be sacred to someone). While you might expect a preposition like für, German often uses just the dative case with certain adjectives to express to whom a statement applies or who holds a certain perspective.

Why is there a definite article (Der) before Sonntag?

In German, days of the week, months, and seasons take a definite article when used as the subject of a general statement. Here, Der Sonntag refers to the concept of Sunday, or Sundays in general.

Could I change the word order to put ihr at the beginning?

Yes! You could say Ihr ist der Sonntag heilig. Placing the dative object in the first position is perfectly grammatical and shifts the emphasis to the person: 'To her, Sunday is sacred (even if it isn't to others).'

Why doesn't the adjective heilig have an ending to match the masculine noun Sonntag?

Because heilig is acting as a predicate adjective, meaning it describes the subject but is separated from it by a linking verb (ist). In German, adjectives only take declension endings when they are placed directly in front of the noun they describe.