The Evening Prayer:
The Hebrew “Tifilat Arevit” (also called “Ma’ariv) refers to the evening recitation of the Amidah (or “Shemonah Esrei”).
Judaism has three services each day, centered on the recitation of the Amidah. They are the morning service (“Shacharit”), the afternoon service (“Minchah”), and the evening service. The afternoon and evening services are usually combined in order to spare participants additional trips to the synagogue. Shabbat and Biblical Holidays have addition Amidot (the plural form of “Amidah”). At the time of Rabban Gamaliel and Rabbi Yehoshua, the morning, afternoon and Shabbat/Holiday additional prayers were all considered obligatory, but it was not yet clear whether the evening prayer would also be considered an obligation or left to the discretion of the worshiper.
The Talmud questions the origin of the prayer services. According to one tradition, each of the three daily services was instituted by one of the three patriarchs; Avraham, Yisthak (Isaac) and Yacov (Jacob). According to this tradition, the evening prayer carries equal weight with the other two; i.e., it is obligatory. Evidently, this is the view of Rabban Gamaliel According to a rival perspective, the prayer services were instituted in parallel to the daily sacrifices offered in the Temple. Only the morning and afternoon services are obligatory, because these had corresponding sacrifices offered daily in the Temple. Following this perspective, the evening service would be voluntary because, in the Temple ritual, there was no actual sacrifice in the evening; merely the remaining parts of the sacrificial animals were burned up. This, evidently, is the view of Rabbi Yehoshua.
Historically, the view championed by Rabbi Yehoshua is the more accurate. Rabban Gamaliel and Rabbi Yehoshua are early enough authorities to have been aware of this. Why would Rabban Gamaliel prefer the historically more farfetched account? Perhaps Rabban Gamaliel had unstated reasons for wanting to impose three obligatory services per day or perhaps the ambiguity of a “voluntary” service concerned him. Alternatively, perhaps he felt that linking the services to the patriarchs provided a firmer foundation for the continued relevance of the prayers in the aftermath of the Temple’s destruction.