somehow the idea of 6 creative days of 6000=42 000 + 7000 of a rest, Sabbath day appealed to the numerology minded wt writers, it fitted with the OT Jubilee cycle of - 49 +1 year. Subdividing the the last 7 millennia further into an 6 work +1 day of rest and judgement by "jesus". This fitted the overall pattern, and the 1 day= 1000 year rule. It allowed too to re-inforce the idea that after the end of the 1000 year kingdom would follow another millenium "day" to complete the type of the 50 year jubilee cycle. since seven is a spiritually perfect number, it makes perfect sense. of course Satan the talking snake has to raise it's ugly head in the 50th day again, so, no peace for the wicked. time out.
P.S. wt: the creative process of the earth, including the "new creation" is EONS long.