When a bris or a pidyon haben occur on erev Tishah beAv, the festive meal should be held before noon.

You should not go for a walk on erev Tishah beAv; and in the afternoon, it is the custom to study only the subjects you are permitted to study on Tishah beAv.¹ cites other opinions who maintain that this custom is an extraordinary stringency, and concludes that we need not protest if someone maintains his usual course of studies, even on erev Tishah beAv. He cites the opinions of Rashal, Vilna Gaon, Chayei Adam, who permitted regular study of all subjects. (Mishnah Berurah 553:8)

footnotes: ¹ Mishnah Berurah

Concerning the final meal before the fast, there are many laws. The correct custom is to eat a regular meal before the Minchah service. After that, you pray Minchah, omitting Tachanun, because Tishah beAv is called a Moed as it is written, "He has proclaimed a Moed for me" (Lamentations 1:15). Towards evening, you should sit on the floor² 552:18) but it is not necessary to take off your shoes. Three people should not sit down (to eat) together, so that they will not be required (to recite Birkas Hamazon) with zimun. You should eat only bread and a cold hard-boiled egg.³ does not mention that the egg must be cold. You should dip a small piece of bread in ashes and eat it.⁴.” (Ibid. 552:16) You should be careful to finish the meal while it is still day. (Whether you are permitted to eat after this, see 131:12 below).

footnotes: ² A weak person is permitted to place a cushion or mat on the floor to sit on after concluding this meal. It is permitted to sit on a regular chair until nightfall. (Ibid. ³ Mishnah Berurah ⁴ Before eating it, you may say, “This is the meal of Tishah beAv

If a person fasts throughout the year on Mondays and Thursdays, and erev Tishah beAv occurs on either of these days, he should have his vow annulled. If someone observes Yahrtzeit on erev Tishah beAv, he should resolve on the first Yahrtzeit, to fast only until noon has passed, then he should pray Minchah gedolah (early Minchah), that is, half-an-hour past noon, eat a meal, and afterwards towards evening, he should eat the final meal.

During twilight, you are forbidden to do all that is forbidden on Tishah beAv. You must, therefore, take off your shoes before twilight.