What should I do, as I have vowed to read one juz' of the Quran daily, but missed a day due to being busy?

Many scholars disliked making vows, especially conditional and repetitive ones, because they can lead to hardship. A vow tied to a specific time must be fulfilled at that time. If one delays fulfilling it, it becomes due as a make-up. If one delays it without an excuse, they are sinful and, according to the Hanbalis, must additionally offer an expiation for a broken oath, besides making it up.