The Torah states that "all labor shall not be done" on the festival days of Passover. The Mekhilta reads this straightforwardly — it tells us that labor is forbidden on the first and last days. But what about the intermediate days, chol ha-moed? The verse says nothing about them.
Rabbi Yoshiyah finds the answer in a different verse. (Exodus 23:15) commands, "The festival of matzot shall you keep seven days." The word "keep" — tishmor in Hebrew — implies guarding, protecting, maintaining the festival character of all seven days without exception. Rabbi Yoshiyah reads this as extending the prohibition of labor, in some form, to the intermediate days as well.
The phrase "in any event" is crucial. It signals that regardless of how one interprets the specific restrictions of the first and last days, the seven-day commandment establishes a baseline of observance that covers the entire week. The intermediate days cannot be treated as ordinary workdays.
This debate had enormous practical consequences for Jewish life. Farmers, merchants, and artisans needed to know whether they could work during chol ha-moed. The eventual legal tradition developed a nuanced middle position: certain types of labor are permitted during the intermediate days (especially work that prevents financial loss), while others remain forbidden. But the foundation — that chol ha-moed carries real restrictions — traces back to readings like this one in the Mekhilta.