Can one be tovel in the mikvah Shabos afternoon as hachona for Purim Motzei Shabos.
Or would this be considered as some kind of hachona l’chol.
Although some authorities permit a person to immerse in the mikva on Shabbos before Yom Tov, others prohibit this, and for Purim it would not be proper practice. There would be room to be lenient to immerse fifteen or eighteen minutes into bein hashemashos on Motzaei Shabbos.
Sources: Me’or Ha-Shabbos (vol. 2) quotes from Rav Shlomo Zalman zt”l that there is no prohibition on a “spiritual preparation,” but quotes from others (Kenei Bosem) that it is prohibited because of hachanah. Emek Teshuvah (3:14) writes that this is not considered preparation, because the tevillah takes effect immediately, but this would probably apply only to tevillah on Shabbos or Yom Tov eve, which has special effect according to kabbalah, and not to tevillah before Purim, for which I am not aware of a source. Maharshag (2:155) writes that the prohibition of hachanah does not apply when there is no other way to prepare for after Shabbos (he discusses tevilah of a woman who will not be able to go to the mikvah on Motzaei Shabbos), but this is also difficult to rely on in our case. However, see also Nishmas Shabbos, and based on the lenient opinions, there is room for leniency after the earliest tzeis hakochavim has passed.