The seven glyphs initiating each day of the week all - I think - stand for showing that a new ruling 'planet' now starts.
The disorderly behaviour of Mercury perhaps is illustrated also in the glyph above. The internal markings of the rectangle standing on its end are pointing downwards, something negative.
Compare this with the similar glyph announcing Sunday, where they are pointing upwards:
Mercury is the opposite of the sun. Nobody with eyes can miss the sun, but to observe Mercury is difficult indeed.
In Polynesian thinking the Sun creates order, rules, rationality and logic. This kind of stability is the opposite to the unruly behaviour of Mercury.