וְיֵ֞שׁ אֲשֶׁר־יִהְיֶ֤ה הֶֽעָנָן֙ מֵעֶ֣רֶב עַד־בֹּ֔קֶר וְנַעֲלָ֧ה הֶֽעָנָ֛ן בַּבֹּ֖קֶר וְנָסָ֑עוּ א֚וֹ יוֹמָ֣ם וָלַ֔יְלָה וְנַעֲלָ֥ה הֶעָנָ֖ן וְנָסָֽעוּ׃
And sometimes the cloud was from evening until morning; and when the cloud was taken up in the morning, they journeyed; or if it continued by day and by night, when the cloud was taken up, they journeyed.
Read Kabbalistic (mystical) interpretations of Numbers 9:21: Zohar and classical Jewish mystical sources, free online.