Ideas:

1. This type of glyph I guess is meant to show the concept of a night.

2. Counting these nights in the Mamari Moon Calendar I arrive at 28. But I think that we should add the glyphs showing the full moon and the one just before that:

and then the number reaches 30.

3. As the cycle of the Moon is 29.5, though, I guess that every other month we should skip one of the nights. But I do not - as yet - know which.