128:14 On the day before Rosh Hashana, the practice is to fast until after mincha, then to eat something so that he will not enter the holiday in discomfort. One should spend his day immersed in Torah study, mitzvos and repentance, especially for sins committed against one’s fellow man. One should not wait until the day before Yom Kippur to ask others for forgiveness; he should do it on the day before Rosh Hashana.
128:15 We wash our clothes and cut our hair on the day before Rosh Hashana in honor of the holiday. Hair should be cut before midday. One immerses in a mikvah (no earlier than an hour before midday – Mishnah Brurah 581:26.) We wear Shabbos clothes on Rosh Hashana to demonstrate our confidence in G-d’s kindness that He will bring us through our judgment favorably. (We should not dress too lavishly, however, because of the seriousness of the day of judgment – Mishnah Brurah 581:25.)