No ika sign can be found in connection with the other days of the week, not even in any of the other 'calendars' which I have tentatively identified as referring to the planets.

That should not be surprising, because the main point with planets seems to have been not whether they are in ascending of descending phase, but how they look and which stars are near.

The planets move around the 'waist' of the sky, along the ecliptic, in a way on the 'surface of the water' - i.e. they are not fish-like. However, the association between ika and Saturn is confirmed in what I believe to be another calendar for the week:

Here the fish signs are not small (as in Hb9-53), and Saturn may be connected with ika also conceptually (not only because it may have been named Ikaika). In the Swedish language Saturday is Lördag meaning the day to bathe and clean oneself (horoi in Tahitian).

I guess the explanation here furthermore hinges on Saturday being the last day of the week. To express the notion of a fortnight two fishes could have been used. The wedge signs on the fish heads may refer to the month, and the statement could therefore possibly be read: 'there are 4 weeks in a month'.