The Talmud teaches that all who need immersion (tevilah) for purification may do so during the day, with the exceptions of the nidah and the yoledet, who immerse at night. The Talmud provides Scriptural basis for this. R. Herschel Schachter (B’Ikvei haTzon 38:7), citing R. Soloveitchik, explains this distinction as a function of the proper application of the principle of “miktzat ha-yom k’kulo” (a part of the day is considered as the whole day; Moed Katan 19b), which allows the last day of the process to “end” while it is still daytime. This principle is only relevant to a process that requires actions over a period of time, and not merely the passage of time itself. Since that action can be done during the last day with time remaining, the process can be said to be complete. However, if all that is required is for the day to pass, then it is necessary for the entire day to pass. This distinction is relevant to the application within the sphere of aveilut (mourning) as well. Miktzat ha-yom k’kulo is applicable to shivah and to shloshim, but not to the twelve month mourning period for a parent. This is due to the fact that the first two are accompanied by active practices of grief. The last, however, is merely a term of abstention from certain expressions of joy, and thus is not subject to miktzat ha-yom k’kulo. As R. Schachter notes as well (ibid, 17), the application of miktzat ha-yom k’kulo to issues of mourning and impurity as opposed to, for example, Shabbat and Yom Tov, can be explained by two other attributes of miktzat ha-yom k’kulo: a) it is utilized only for areas that contain undesirable aspects to them, rather than those of holiness and happiness (Gilyonei HaShas, Nazir 5b, citing Rama MiFanu), and b) miktzat ha-yom k’kulo is not applied to calendaric dates, but only to individual countings that are superimposed upon the calendar.