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1. It is a mitzva to wash the head and cut the nails on Erev Shabbos (שו''ע או''ח סי' ר''ס ס''א ). Some say the reason is to not enter Shabbos unkempt (ביאור הגר''א ); others say it is a fulfillment of oneg and kevod Shabbos to beautify oneself in honor of Shabbos (ס' הבתים פ' הקידוש והשביתה שער ט', י''א ).
2. Every erev Shabbos. Some say it is a mitzva to cut the nails every erev Shabbos (ס' היראה, ט''ז סק''א בשם מהרש''ל ), as implied by the Mechaber placing it together with washing the head, which is a mitzva every erev Shabbos (חוט שני שבת פ''ה ). Others say the point is that when one wants to cut his nails, he should only do it on erev Shabbos or erev Yom Tov, having in mind the honor of Shabbos rather than his personal benefit (ס' המשכיל ). Some say one should at least cut them every other erev Shabbos (בן איש חי ש''ב לך לך, י''ד ).
3. Erev Yom Tov. Similarly, nails that need to be cut should be cut on erev Yom Tov (מטה אפרים סי' תרכ''ה י''ג ). If one cuts them on erev Yom Tov, he may also cut them on erev Shabbos Chol HaMoed, but if he did not cut them on erev Yom Tov, he may not cut them on erev Shabbos Chol HaMoed (see below 39).
