Ideas:

1. But if the week is a mirror-image of the year (or for that matter: of the day), then the middle part ought to have more light than what you can measure in this glyph (and in the identical glyphs in Tuesday and Wednesday). Does the size of this glyph not relate to the amount of visual light? The pattern in this (presumed) Keiti-calendar is:

2. Perhaps it is not the amount of visual light but the amount of magic power (a kind of light?). As when we approach the midwinter solstice and the sun moves slower and slower - a sign of tiredness because it is close to death - the midsummer solstice also announces a change in the ruling power. The magic power of life (vitality) is low. During the hottest time of the day it is best to sleep (moe) and during the hottest time of the year people also are getting tired.

3. Should this type of glyph here mean the measure of visual light, then the middle part of the week must be compared to midwinter (and midnight). I think that fundamentally this type of glyph means light, but that in this calendar we should instead ponder the possibility of it meaning life. Measuring life it would not be strange if the equinoxes (and sunup /sundown) were regarded as most intense, as the sun there moves at its quickest and the changes are most noticeable.

4. The week perhaps is not an image of more than that part of the year which is most interesting for a farmer, the equinoxes and the summer part in between.

5. Another possibility is that there is no intention of measuring amount of light (or life) in the height of the glyph. Instead there is only an intention to lift up the glyphs in the middle part of the (assumed) week. An intention to indicate lesser light or life would better have been expressed with small glyphs placed in the horizontal middle of the textline (or at the bottom), not with small glyphs placed at the top.

6. As Mercury = quicksilver, where quick = moving = full of life, that seems to rule out that the middle of the week should be seen as a period of standstill. On the contrary.

7. The conclusion seems to be that we must try to understand what makes the middle of the (presumed) week higher placed than the rest of the week. One possible explanation could be that the story of the Stranger King lies behind. There is Rangi in the middle of the week.

8. The sun climbs high in the middle of the day and in the middle of the year. The rest of the 'planets' should follow suit and also be at their highest in the middle.