The time between Shiva Asar B’Tammuz and Tisha B’Av is referred to as the period "within the straits" (bein hametzarim), in accordance with the verse: "all her oppressors have overtaken her within the straits" (Aicha 1:3).
During this time no weddings are held, we do not listen to music, or attend public celebrations. We also avoid pleasure trips or other unusually entertaining activities, do not shave or cut hair, and only recite Shehechiyanu for new foods or clothing on Shabbos.
This is also a time when the Divine attribute of judgment is felt so one should be careful to avoid dangerous undertakings.
On the fast day (17th of Tammuz) itself, bathing is permitted but it has become customary not to take a hot shower or bath. It is also proper for adults to refrain from swimming.
Some poskim permit rinsing the front part of the mouth, taking care that no water enters the throat area, while others allow this only when in distress (tza'ar). Accordingly, one should not make a dentist appointment for the fast which would require him to rinse, unless he is in pain.
Prescription medications may be taken with water, if necessary, but if one needs Tylenol or aspirin for a severe headache, he must ruin the taste of the water first.
