Much as we are obligated to begin our day with ritual washing of the hands, in the Temple the kohanim were obligated to wash their hands and feet every morning before they began to perform the sacrificial service. They did this by placing their right hand over their right foot and their left hand over their left foot and washing them from the kiyor – the water basin in the Temple – which was made of two halves, the basin and it base. During the Second Temple, twelve faucets were added to the kiyor in order to allow many kohanim to prepare themselves for service at the same time.
Once the kohen prepared himself by washing in the morning, he can continue performing the Temple service into the night without washing them again. According to Rabbi Yehudah ha-Nassi, if he washed his hands and feet at night, then when the next morning comes he will have to wash his hands and feet again.
Rabbi Yohanan argues that if the kohen were to wash prior to performing the terumat ha-deshen, then he will not have to wash his hands again when the daily sacrifices begin.
The terumat ha-deshen was a unique mitzvah that was performed in preparation for the new day of the sacrificial service in the Temple (see Vayikra 6:1-3). Essentially, it was the opening ceremony for the sacrificial service of the day. This commandment – which involved having one of the kohanim remove from the altar a small amount of ash from the remaining sacrifices that were burned overnight – was performed before dawn, and, during holidays when many sacrifices were to be brought, even before midnight. Only after this symbolic cleansing of the altar took place could the otherkohanim begin to clear the altar and prepare it for the new day.
Abayye explains that according to Rabbi Yohanan, Rabbi Yehudah ha-Nassi’s requirement to wash at the beginning of the day in cases where the kohen has already prepared himself the day before is only a rabbinic requirement. Since theterumat ha-deshen essentially begins the service of the new day, the Rabbis did not require a second washing in this case.