תמימת תהיינה [SEVEN WEEKS] SHALL BE COMPLETE — This teaches that one has to begin counting in the evening, for otherwise they would not be complete (Menachot 66a).

Rabbi Akiva says that the verse should be understood as follows: If one studied Torah in his youth he should study more Torah in his old age; if he had students in his youth he should have additional students in his old age, as it is stated: “In the morning sow your seed, etc.” They said by way of example that Rabbi Akiva had twelve thousand pairs of students in an area of land that stretched from Gevat to Antipatris in Judea, and they all died in one period of time, because they did not treat each other with respect. And the world was desolate of Torah until Rabbi Akiva came to our Rabbis in the South and taught his Torah to them. This second group of disciples consisted of Rabbi Meir, Rabbi Yehuda, Rabbi Yosei, Rabbi Shimon, and Rabbi Elazar ben Shamua. And these are the very ones who upheld the study of Torah at that time. Although Rabbi Akiva’s earlier students did not survive, his later disciples were able to transmit the Torah to future generations. With regard to the twelve thousand pairs of Rabbi Akiva’s students, the Gemara adds: It is taught that all of them died in the period from Passover until Shavuot. Rav Ḥama bar Abba said, and some say it was Rabbi Ḥiyya bar Avin: They all died a bad death. The Gemara inquires: What is it that is called a bad death? Rav Naḥman said: Diphtheria.

It is customary not to get married between Pesach and Shavuot, until Lag BaOmer (the 33rd day), because during that time, the students of Rabbi Akiva died. However, to do "erusin" and "kiddushin" (engagement and betrothal) is OK. And even for "nisuin" (marriage), if someone did so, we do not punish him. Rema: however, from Lag Ba'Omer onwards, all this is permitted (Abudraham, Beit Yosef & Minhagim). It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying. One should not cut one's hair until the 34th day, in the morning, unless the 33rd day falls on Friday, in which case one may cut one's hair then, in honor of the Sabbath ("kavod Shabbat"). Rema: But in these countries, we do not follow the custom he advocates; rather, we cut our hair on the 33rd day, and we rejoice a bit, and we do not say Tachanun (Maharil and Minhagim). And one should only cut one's hair on the 33rd day itself, but not on its evening. However, if it falls on Sunday, our custom is to cut our hair on Friday in honor of the Sabbath (Maharil). Someone who is performing a brit milah (i.e., the sandak, mohel and the infant's father), or circumcizing his son, is allowed to cut his hair during sefirah in honor of the circumcision (Hagahot Minhagim). Some have the custom to cut their hair on Rosh Chodesh Iyyar, but this is a mistake. Rema: However, in many places, the custom is to cut their hair until Rosh Chodesh Iyyar, and they do not cut their hair from Lag BaOmer forward, even though it is permitted to cut one's hair on Lag BaOmer itself. But those places who have the custom to cut their hair from Lag BaOmer onwards, do not cut at all after Pesach until Lag BaOmer. And within one city, it should not be that some follow one custom, and some follow another, because of "lo titgodedu" ("do not cut yourselves" - Deut. 14:1, interpreted in Yevamot 13b as "lo te'asu agudot agudot," "Do not become seperate groups"); all the more so, one may not follow both leniencies. Women have the custom not to do work from Pesach until Shavu'ot, from sunset onwards.