Excerpt from Rabbi Bezalel Naor’s Haggadah “Springtime of the World”:
The word kadesh (sanctify) is in the singular. The lesson to be derived from this grammatical form is that though the sanctification of time is a collective event on the part of the Jewish People, nevertheless the individual is not obliterated in the collective. Rather, each individual contributes a unique dimension. Israel is not like some totalitarian regime where the individual becomes subjugated to the collective. On the night of this festival, which is the foundation of all the appointed times of the Lord, each member of Israel has the ability to bestow upon the day his or her very own dimension of sanctity. This individuality is part and parcel of our emancipation from Egyptian bondage. Each individual Jew is called upon to recognize the worth of his contribution. “Kadesh! Sanctify! (singular)."
(Rabbi Abraham Isaac Hakohen Kook, Siddur 'Olat RAYaH II, pp. 254-255)