105 kin is too long a period, though, not to be subdivided. I imagine that therefore originally the idea was that there were 15 weeks with 7 kin in each week during this period. Which means that they had two types of week: during the hottest period weeks with 7 kin and during the rest of the year weeks with 20 kin.

13 weeks with 20 kin + 15 weeks with 7 kin gives 28 weeks in a year, i.e. the same number as the number of nights when the moon is visible in a month. Such facts are important. The alternative: 13 + 7 (with 15 kin) = 20 weeks might seem less beautiful, but considering Venus - see below - is also possible.

The moon is the primary time giver and the 7 'planets' are also important. What about 20 and 13? The number 13 we have already discussed. It is - among other things - a consequence of dividing 364 by 4, which results in 91 days per quarter = 7 * 13 days.

20 is easily accepted as that is the number of fingers and toes. Perhaps too easy an explanation though, because Venus returns to the same 'house' in the sky after eight years and she has 5 such houses, i.e. 8 * 5 = 40 (= the number in my idea of a basic period of time which can be folded into two 20-day weeks). Olympic games are held every four years.

And we should not forget the meetings between Jupiter and Saturn.