Mekhilta is one of the halakhic midrashim. The name itself “Mekhilta” is an Aramaic word meaning “rule” or “norm.” It is used in the Talmud to designate not the commentary specifically but general notes on halakhic exegesis and the rules guiding that exegesis (ITM, 252). This is common in rabbinics - “midrash” can refer to a book like Mekhilta or simply to an exegetical method; “mishnah” can refer to the Mishnah or to a particular law/section in the Mishnah. The Mekhilta is named after R. Ishmael, the first authority named in Pisḥa 2. The exegesis covers Exodus 12:1-23:19; 31:12-17; and 35:1-3 (ibid.). As with much rabbinic literature, pinning down a precise date of composition is difficult. It is one of the tannaitic midrashim, containing early rabbinic traditions and exegesis. It was probably redacted sometime in the late 3rd century or 4th century C.E. in Palestine.
The Hebrew text below is from Lauterbach's edition. The English translation is mine.
Pisḥa 1, Parashah 1, lines 1-10.
ויאמר יי אל משה ואל אהרן בארץ מצרים לאמר שומע אני שהיה הדיבור למשה ולאהרן כשהוא אומר ויהי ביום דבר יי אל משה בארץ מצרים למשה היה הדיבור ולא היה הדיבור לאהרן אם כן מה תלמוד לומר אל משה ואל אהרן אלא מלמד שכשם שהיה משה כלול לדברות כך היה אהרן כלול לדברות ומפני מה לא נדבר עמו מפני כבודו של משה נמצאת ממעט את אהרן מכל הדברות שבתורה חוץ משלשה מקומות מפני שאי איפשר׃Translation
“And the Lord said to Moses and Aaron in the land of Egypt, saying” (Exod. 12:1). I could understand that the divine revelation (הדיבור; Jastrow, 295) was for Moses and for Aaron. But when it says, “And it came to pass on the day when the Lord spoke to Moses in the land of Egypt” (Exod. 6:28), the divine revelation was directed to Moses and not to Aaron. If thus, what is being taught by saying “to Moses and to Aaron”? It only teaches that just as Moses was included for the divine words, so Aaron was included for the divine words. So because of that, why does he not converse with him? On account of the honor due Moses.The rabbis here are noticing that sometimes the biblical text depicts God speaking only to Moses and other times mentions Moses and Aaron together. The discussion continues on to the issue of whether word order signifies priority and importance, but we'll get there next. The observation here is that Moses is deserving of more honor and respect which is why God spoke to him first. I'm unsure of the nuance where I've translated “You will find Aaron excluded”. I think Lauterbach has a more accurate assessment of the context when he translates “Aaron was not directly addressed” (p. 1), intimating that while Aaron was there and included, he was never directly addressed except three times. In these three cases, it's impossible to find anyone except Aaron as the direct addressee: Lev. 10:8, Num. 18:1, and Num. 18:8. Next up, Mekhilta on word order and equality, continuing on in parashah 1.
You will findConsequently, [Scripture] excludes Aaron from all the divine revelations in the Torah except for three places where it is impossible.
Boyarin, D. Intertextuality and the Reading of Midrash. Indiana University Press, 1990.
Jastrow, M. Dictionary of the Targumim, the Talmud Babli and Yerushalmi, and the Midrashic Literature. Putnam, 1903.
Lauterbach. J.Z. Mekilta de-Rabbi Ishmael: a critical edition on the basis of the manuscripts and early editions with an English translation, introduction and notes. JPS, 1961 .
Strack. H. and G. Stemberger. Introduction to the Talmud and Midrash. Fortress Press, 1996.