69:2 Before we discuss the time for Mincha, let us define the term "hours." In our context, "hours" means "seasonal hours" ("shaos z'maniyos" in Hebrew). The length of these hours is calculated by dividing the daylight hours, from first light until sunset, into twelve equal portions. Accordingly, shaos z'maniyos are longer in the summer than in the winter. That having been established, the optimum time for Mincha begins at nine and a half hours, which is called "mincha katana" (small mincha). In compelling circumstances, such as if one needs to depart on a journey (or if one has a minyan now but may not have one later – Mishnah Brurah 233:1), one may recite Mincha after six and a half hours, which is called "mincha gedolah" (large mincha). (Some authorities permit one to daven at mincha gedolah even at the outset – MB 233:1.) Mincha should only be recited until an hour and a quarter before nightfall, which is called "plag hamincha" (mincha midpoint). Plag hamincha is the midway point between mincha ketana and nightfall, which is two and a half hours. After the fact or in a case of need, one may recite Mincha after sunset. (The Kitzur Shulchan Aruch says "until the stars come out," but he is not speaking literally – see MB 233:14.) The commonly-accepted practice nowadays is to recite Mincha shortly before evening.
69:3 One is not permitted to start eating even a small meal within a half-hour of the time of mincha ketana. There are authorities who permit one to have a snack (even after the time for Mincha has arrived – MB 232:34). The Kitzur Shulchan Aruch advises acting stringently in this matter but the Mishnah Brurah appears to allow it at the outset (see MB 232:34-35.) Similarly, one should not enter a bathhouse or start a haircut within a half-hour of mincha ketana. One may not begin a large meal, such as those for a wedding or a bris, close to the start of mincha gedolah, i.e., midday. (Mincha gedolah is 6½ hours, so a half-hour before is chatzos – halachic “noon.”) One should wait until the time of the mincha gedolah, daven, and then begin the meal. In a place where they have a regularly-scheduled minyan that one usually attends, one may begin a small meal close to the mincha ketana - and even after the time for Mincha has arrived - provided that one stops when it is time for the minyan. One may not begin a large meal close to mincha ketana even if there is a regularly-scheduled minyan. The Kitzur Shulchan Aruch advises acting stringently in the matter of a large meal even close to mincha gedolah.