A familiar function of grammar in all languages
is the use of the construct. The construct
has a variety of meanings including possession and
association. The construct can also dynamically
create new meanings.
Rashi interprets numbers in the construct as indicating
groups. Thus three days becomes, when the construct
is used, a triplet of days. Similarly 7 days
becomes, when the construct is used, a sextuplet of days.
Following Rashi we have the translations of the
following two verses:
And Moses stretched forth his hand toward heaven; and there was a thick darkness in all the land of Egypt for a triplet of days;
For a septuplet of days ye shall eat unleavened bread; howbeit the first day ye shall put away leaven out of your houses; for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from I
In both cases the use of construct with number is interpreted
by Rashi as indicating grouping.