It is permissible to wash one’s hands for the sake of a mitzvah, because it is not for pleasure purposes. Therefore, kohanim (“priests”) may wash their hands fully, in preparation for the priestly blessings (Rama 613:3, Sh.A. 128:6). However, one is not permitted to immerse in a mikveh on Tish’a B’Av.
Upon awakening in the morning, everyone is obligated to wash his or her hands up until the joints connecting the fingers to the palm of the hand, because an evil spirit rests on one’s hands after a night’s sleep, and it can cause harm to the orifices of the body. In order to remove this spirit, one must wash each hand three times, alternately. After using the bathroom, one should wash his hands again, once, and recite the blessing Al netillat yadayim, because the Sages instituted a mitzvah to wash one’s hands, with a blessing, in preparation for the morning prayers (Shacharit). And even though we are usually careful to wash the entire hand, on Tish’a B’Av one should wash only up to the joints connecting the fingers to the palm, because according to the letter of the law, that is sufficient both in terms of preparing for Shacharit and in order to remove the evil spirit (Sh.A. 613:2).
Throughout the year, one should preferably wash his hands three times before every prayer service. Nevertheless, on Tish’a B’Av, one should not wash his hands before praying, because doing so is not obligatory. However, one who touched filthy parts of his body and wants to recite holy words should wash his hands, because he is doing so for the sake of a mitzvah, not in order to derive pleasure (M.B. 613:5-6, K.H.C. 6).
There is uncertainty regarding the law of someone who relieves himself without touching any part of the body that is usually covered, for perhaps he does not need to wash, seeing that he did not touch any filth. In order to avoid the quandary, it is best to touch a usually-covered part of the body – which have flecks of sweat – when using the bathroom. This way, everyone would agree that one may wash his hands until the upper knuckles, in order to say the blessing of Asher Yatzar in cleanliness (Sh.A. 613:3, M.B. 4).