Ideas:

1. Here the thickness of the glyph presumably carries meaning. Friday and Saturday show this sign. The preceding days are lean.

2. The number of internal marks are 4, also making a connection with Saturday evident. Because 4 + 3 = 7.

3.Let's compare the 'Clara-knife' in Friday with those of the other days:

unique	similar		similar		unique	unique

The 7 days of the week result in 5 groups. And 5 is the number of the fingers and also of the 'homes' of Venus: only 5 of the 12 constellations in the zodiac are visited by Venus (Taurus, Sagittarius, Cancer, Aquarius, Virgo).