To ensure the recital of the Minchah prayer, the Chachamim instituted that from the time of chatzot onward, a person must be careful not to begin activities that will likely cause him to be so distracted that he will forget to pray. Therefore, one may not start work that will be difficult to stop in the middle, and will likely continue past the end of the time to pray. Likewise, he may not begin work that might create a problem which, in order to fix it, would require continuing past the final time to pray. One may not shop if the shopping may last until after the final time to pray. Similarly, one may not swim in a pool when there is concern that he might remain there until the time to pray ends.
However, in a situation in which there is no concern that he will forget to recite Minchah, these activities are permitted. For instance, a person is permitted to start any type of work if he is in a place where he will be called to pray Minchah. Similarly, one is allowed to begin all kinds of work when he knows that the workplace will close before Minchah time passes. It is also permissible to shop in a place where they call people to pray Minchah in a minyan, or when one asks his friend to remind him to pray Minchah. Likewise, it is permissible to swim in a place when the pool is closed before the time to recite Minchah lapses; or when a person normally swims at a certain set time, and there is no concern that he will be tempted to stay past Minchah time.
If a person began doing something that is likely to continue a long time, and he estimates that he will finish before the final time to recite Minchah, since he already began the activity, he is not required to stop in the middle. Instead, he prays upon the conclusion of his activity. However, if it seems to him that what he is doing will continue past the end of Minchah time, he must stop immediately and pray.
Although the Chachamim forbade getting haircuts at the time of Minchah, nowadays the Acharonim permit cutting one’s hair even after the time to recite Minchah has begun (Mishnah Berurah 232:6; Kaf HaChaim 14). This is because haircuts today are completed in a very short amount of time and there is no concern that perhaps a problem will occur with the scissors or the shaver, since every barber has several haircutting kits.
. See Shabbat 9b, where it is written that it is forbidden to get a haircut, enter a bathhouse or a tannery, eat, or judge a case of law close to Minchah time. The Rishonim are divided as to whether or not Chachamim intended to forbid a significant act like a long bathing or a large feast, or even a minor act, such as a short bath or a small meal, the reason being that small acts lead to bigger ones, like a short meal eventually becoming a long one. Further, they disagree whether the prohibition begins at chatzot (halachic noon) – half an hour before Minchah Gedolah, or starts after nine hours have passed – half an hour before Minchah Ketanah. The Rif, Rambam, and Shulchan Aruch rule stringently concerning performing even a minor act near the time of Minchah Gedolah (Shulchan Aruch 232:2). Rabbeinu Tam and the Rosh forbid a significant act near the time of Minchah Gedolah and a small act near the time of Minchah Ketanah (according to the Gra, this is the main and median opinion). The Maor and the Rashba maintain that both significant and minor acts are only forbidden near the time of Minchah Ketanah. Rabbeinu Yonah maintains that a minor act is always permissible and a major act is forbidden near Minchah Gedolah. The Hagahot Mordechai and the Mahariv are more lenient and maintain that only a significant act is forbidden from close to Minchah Ketanah, but a minor act is permissible at all times. (The Rama 232:2 writes that we practice this way. However, it is proper to be stringent concerning a large feast from chatzot, like the opinion of Rabbeinu Tam. Regarding this entire matter, see Rama 232:2, Mishnah Berurah 232:5 and similarly, paragraphs 21-26, as well as Aruch HaShulchan 8-16.) Bei’ur Halachah s.v. “Labursiki” writes that all activities which will likely last a long time are forbidden, like the ruling about going to a tannery.
However, the Acharonim wonder why nowadays people are not careful concerning these matters. Aruch HaShulchan 15 writes that the cause for leniency lies in the Yerushalmi. The main point, it seems, is that living conditions and types of work have changed, and therefore it is hard for us to establish the halachah according to the limitations of the activity and the times. Instead, it is necessary to return to the basic rule, namely, not to start something that might distract one’s mind until Minchah time lapses, and the more bothersome the activity and the closer it is to the end of Minchah time, the more need there is to be careful. In a place where there is someone who can remind the people to pray, there is no concern, as the Rama 232:2 says. Even when there is no such person, the Aruch HaShulchan 232:16 writes that someone who is accustomed to praying in a set minyan, and normally stops in the middle of his work or business, may start any activity, for there is no concern that he will forget to pray. Today, we rely on his reasoning.