Is Ruhetag der, die or das?

The correct German article for Ruhetag is der. The noun Ruhetag is Masculine. So you always say: der Ruhetag. Learn the article together with the noun — German nouns don't follow predictable gender rules most of the time.

The plural of Ruhetag is: die Ruhetage. In the plural, every German noun takes the article "die". So: die Ruhetage. Example: ein Ruhetag → zwei Ruhetage.

Correct: die Ruhetage. The form "die Ruhetags" is wrong. The plural of der Ruhetag is die Ruhetage. In the nominative plural, you always use the article "die".

der Ruhetag follows the four German cases. Nominative: der Ruhetag, Accusative: den Ruhetag, Dative: dem Ruhetag, Genitive: des Ruhetages. In plural: die Ruhetage / den Ruhetagen. Learn these forms together with the noun.

The genitive singular of der Ruhetag is: des Ruhetages. The genitive plural is der Ruhetage. German uses the genitive to express possession or belonging. Example: die Farbe des Ruhetages.

The dative plural of Ruhetag is: den Ruhetagen. In German, dative plural nouns usually end in "-n". So you say: "mit den Ruhetagen". Use this case after prepositions like mit, von, bei, zu.

Ruhetag is a noun and must be capitalized in German. You'll find example sentences above for each case. The article der stays the same in nominative singular but changes in other cases. Practice declension in your own sentences.

Yes, Ruhetag is a noun (also called a substantive). German nouns are always capitalized. The article for Ruhetag is der. Every German noun has a fixed article you must memorize.