(Exodus 13:10) states, "from day to day" — miyamim yamimah. The Mekhilta asked why this phrase was necessary. After all, the previous verse already established that the account of the Exodus "shall be to you as a sign." If tefillin (leather phylacteries worn during prayer) are a sign, then presumably they should be worn at all times — day and night alike.
And there is a logical argument that would support nighttime wear. Mezuzah (a parchment scroll affixed to doorposts) is a positive commandment. Tefillin are a positive commandment. Both involve affixing sacred texts — one to the doorpost, one to the body. Since a mezuzah remains on the doorpost both day and night, one might conclude that tefillin should likewise be worn both day and night.
The Mekhilta rejected this analogy by pointing to the phrase "from day to day." The repetition of "day" — not "day to night" or "time to time" — restricts the commandment to daytime hours. Tefillin are worn during the day and not at night.
This ruling became normative halakhah (Jewish religious law). The Talmud (Menachot 36b) elaborates on the principle, and the Shulchan Arukh codifies it: tefillin are put on in the morning and removed before nightfall. The phrase "from day to day" serves as the scriptural anchor for this time restriction, distinguishing tefillin from mezuzah and establishing them as a commandment bound to sunlight — a sign worn in the hours of visibility, when signs can be seen.