[Glen's idea of six and seven having special significance, quoted below]
You need to look very carefully at the evidence before making this claim.
In many cases, the significance follows the fact, rather than the fact being
derived from the significance. The Biblical story of creation is presented
as the reason why Hebrews kept the seventh day sacred, but it is probably
the other way round: Hebrews worshipped every seventh day, and then
re-fashioned the creation story to make it fit this fact. We can see how
the Babylonian 8-day version has been squished to a 6-day version (there are
two days on which two events happen, which in the Babylonian version appear
in separate days).
Biblical and Classical material is also very, very much later than the PIE
period. So what evidence do we actually have for the sacredness of seven or
six prehistorically?
Peter
>"six" and "seven" had numerological significance even in
> prehistory. Thus, the ubiquitous adoption of "seven", particularly from a
> Middle-Eastern tongue in many unrelated European and West Asian languages,
> suggests to me that "seven" was something divine, a symbol perhaps of the
> goddess. We in fact later see the use of the numeral as a symbol in
> connection with the Bible (where "seven" represents completion, divinity,
> associated with God, etc as with the "seven days of creation" in
> opposition to the "six(-six-six)" of Satan) and also we hear of the Seven
> Sisters of the Pleiades and seven days of the week. The number of days
> in the year is 365 which when divided by 52 (a multiple of the prime 13),
> gives another prime, 7.