According to IH Marshal (NIGTC, in loc.), 8 days could refer to 1) a
Hellenic week (as opposed to a Jewish 7-day week) or inclusively to a
7-day week (cf. John 20:26). In any case, the "hwsei" prefixed to this
number lets us know that it is only approximate, with the probable
basic meaning of "about a week after 'logous toutous'". To me it seems
reasonable to take "after 6 days" as equivalent to "after
approximately a week."
[Also note that in Greek, the object of the preposition 'after' is
'these words,' not 'about 8 days'. The gramatical construction used
here is explained in Blass & Debrunner (10th ed), s. 144]
Theos Boethos,
Andrew.