Gleason L. Archer, Encyclopedia of Bible Difficulties, pages 60-61, Baker 1982:
“ There were six major stages in this work of formation, and these stages are represented by successive days
of a week. In this connection it is important to observe that none of the six creative days bears a definite article in the Hebrew text; the translations “the first day,” “ the second day,” etc., are in error. The Hebrew says, “And the evening took place, and the morning took place, day one” (1:5). Hebrew expresses “the first
day” by hayyom harison, but this text says simply yom ehad (day one). Again, in v.8 we read not hayyom hasseni (“the second day”) but yom seni (“a second day”). In Hebrew prose of this genre, the definite article was generally used where the noun was intended to be definite; only in poetic style could it be omitted. The same is true with the rest of the six days; they all lack the definite article. Thus they are well adapted to a sequential pattern, rather than to strictly delimited units of time.”
Gleason Archer was Associate Editor of the Theological Wordbook of the Old Testament. In the quote above, the first two italicized letters ha of words like harison indicate the Hebrew prefix “heh” meaning “the.”