Individual kanji have on and kun readings. Which character in a multiple kanji word determines if the word has an on or kun reading? 月曜日 is a better example, on on kun. Would Nichiyoubi be considered an onyomi or kunyomi word?
Most multi-kanji words have either only on-readings or only kun-readings, but there are exceptions. For example, 時計 is neither purely kun nor purely on. (と is a kun-reading, けい is an on-reading)
Words like 時計 are called kun-on compounds or 湯桶読み. Likewise, words like 毎年 is called on-kun compounds or 重箱読み.