"mitmol": two days; "shilshom": three days. As R. Yehudah says: What constitutes a "foe"? One with whom he did not converse for three days out of hatred.
(Devarim 19:5) "And one who comes with his neighbor in the forest": Just as a forest is a place that the slain and the slayer were permitted to enter, so all such places (are subsumed in the halachah) — to exclude the owner's courtyard, which the slayer had no right to enter.