The prevalent minhag is that of the Chachamim, that Minchah time lasts until the end of the day, and the time to pray Ma’ariv begins after tzeit hakochavim (the emerging of the stars) (Berachot 26a). However, according to Rabbi Yehudah, the time of Minchah lasts until plag haminchah. “Plag” means half; in other words, the time of Minchah Ketanah is divided into halves. We already learned (halachah 3) that the time of Minchah Ketanah is from nine-and-a-half proportional hours until the end of twelve hours, a total of two-and-a-half hours, and if so, one plag is an hour and a quarter. The first plag is the time to pray Minchah, and the second plag begins the time of Ma’ariv, according to Rabbi Yehudah.
The reason for his opinion is that, in actuality, the Kohanim would rush to slaughter the Tamid and throw its blood upon the altar in the first half of Minchah Ketanah, and therefore that is the time for Minchah. In the second half they would already start offering the organs of the Minchah upon the altar, and since the Ma’ariv prayer was instituted to correspond to the sacrifice of the organs, it is then that Ma’ariv time begins. According to the Chachamim, since it is possible, in principle, to throw the blood of the Tamid upon the altar until the end of the day, the time of Minchah also lasts until the end of the day. The time of Ma’ariv is coordinated with the time to recite Keriat Shema of Ma’ariv, which begins at tzeit hakochavim.
In practice, this disagreement is left unresolved, and the conclusion of the Talmud (Berachot 27a) is that a person is permitted to decide whether he wants to practice like the Chachamim or like Rabbi Yehudah. However, the Rishonim caution that he must make sure that he consistently adheres to one minhag. In other words, if he follows Rabbi Yehudah, he must take care not to pray Minchah after plag haminchah. If he practices like the Chachamim, he must make sure to always recite Ma’ariv after tzeit hakochavim. However, it is forbidden to pray Minchah after plag haminchah like the Chachamim, and Ma’ariv before tzeit hakochavim like Rabbi Yehudah (we shall expand on this law in the laws of Ma’ariv, 25:6-7).
. The poskim disagree as to how the day is defined. According to the Shiltei HaGiborim, Gra and majority of poskim, it is from netz to sunset, yet the Terumat HaDeshen maintains that the day is from amud hashachar (dawn) until tzeit hakochavim. Based on this, the time of plag haminchah varies. Is it a (proportional) hour and a quarter before sunset, or before tzeit hakochavim? The minhag is to calculate plag hanminchah from sunset and that is what is written in Kaf HaChaim 233:7 and in the Israeli calendar. However, it can be inferred from the Sha’ar HaTzion that l’chatchilah both opinions need to be taken into consideration (233:4; Sha’ar HaTzion 235:14). Still, it is necessary to note that according to the approach of the Terumat HaDeshen one must calculate tzeit hakochavim like Rabbeinu Tam, meaning 72 minutes after sunset, so that chatzot will be in the middle of the day, as explained earlier in this book, chapter 11, note 14. Based on this, plag haminchah is slightly before the time of the visible sunset (between two and eighteen minutes, fluctuating according to the seasons of the year).