One of the activities that were incumbent on the person who brought a sacrifice was semikhah – laying hands on the animal that was to be sacrificed before its slaughter. The Gemara on today’s daf (=page) quotes a tosefta that teaches that tekef le-semikhah, shehitah – that this laying of hands was performed immediately prior to the slaughter of the animal. One exception was the case of a metzorah – someone who had recovered from biblical leprosy and was obligated to bring a number of sacrifices to formally put an end to his ordeal. Since his status as someone who was ritually impure did not allow him to enter the Temple courtyard where the slaughter took place, he would perform it in sha’ar Nikanor – the Nikanor gate – at the entrance to the courtyard.
Sha’ar Nikanor served as the main entrance to the Temple courtyard from the outer precincts of the Temple Mount, at the top of a series of steps leading from the ezrat nashim. As our Gemara explains, one of its purposes was to allow someone who needed access to the Temple courtyard to get as close as possible without actually entering it.
Generally speaking, halakhah considers the entrance to a place to have the legal status of the place to which it leads, but sha’ar Nikanor, which was to the east of the Temple courtyard, was specifically denied the status of the courtyard in order to allow people who were ritually defiled to approach the Temple and perform whatever rituals were necessary. In the case of a metzorah, for example, part of the ritual that would lead to his being readmitted to society was bringing a number of sacrifices, and also having blood from the sacrifice, as well as oil, placed on his right ear, thumb and big toe (see Vayikra 14:10-32). All of these had to be performed in the Temple courtyard, and only by having the metzorah stand in sha’ar Nikanor could this be accomplished.