In NetHack, while starting the game, you will occasionally get a message about the moon phase. NetHack only cares about a full moon or a new moon, and to some extent what time of day it is.

If you are playing while there's a new moon, be careful. There aren't any effects to luck or undead (like with a full moon), but there's one very important thing: if you are fighting a cockatrice, and it hits you and then hisses, and you aren't carrying a lizard corpse, you will turn to stone immediately; do not pass "GO", do not collect $200. Usually, you turn to stone slowly after bad encounters with cockatrices, giving you a chance to cure the petrification, but in this case you die right away. Be careful, and always carry a lizard corpse or two to combat the stoning.