A fish going down. Remember the moon calendar in Mamari: fishes going up until full mon, then fishes going down. Those fishes going down have double wedges marking the gills, the fish above only one. I guess that double wedges are connected with the moon.

There are double wedges in Saturday too, according to Keiti. Although these fishes are going up. As they are two we could assume them to mean 2 * 14 = 28 nights = a month.

A wedge upwards means darkness, I am fairly sure of, e.g. in GD46.