The Torah tells us that Moshe was given an instruction that “On the day of the first month, on the first day, you shall erect the Communion Tent Tabernacle.” (40:2). This was fulfilled in 40:17, “In the first month of the second year [of the Exodus], on the first of the month, the Tabernacle was erected,” as a followup to the verse that informs us that “Moses proceeded to do exactly as G-d had commanded him.” (40:16)
The midrash is fond of the sentiment that the Mishkan was assembled and disassembled on a daily basis over the course of its first week of operation, leading up to the anticipated “Eighth day.” The midrash Tanchuma goes so far to say, based on the three appearances of the verb “KM” (erect, assemble) (40:2,17,18), that this process took place three times a day during days of miluim (the dedication days leading up to the final consecration of the Mishkan).
The language the Torah uses to introduce us to the day the Mishkan was to be erected is strange in two regards. The Ibn Ezra, in his Peirush Ha’Arokh, recounts a debate as to whether the first of Nissan (per the name we now give to the first month) was the first day when the Mishkan was erected, or whether the first of Nissan was in fact the 8th day of the miluim (he notes that if the first of Nissan was the eighth day of the miluim [the main dedication day as noted in Vayikra 9] then the Mishkan was actually first assembled on the 23rd of Adar). Why doesn’t the Torah mention this date? And why doesn’t the Torah mention the daily ups-and-downs of the Mishkan?
Secondly, as Meshekh Chokhmah argues, the phrase “b’yom hachodesh harishon” (on the day of the first month) seems superfluous, since the Torah then says “on the first of the month.” He learns, therefore, that the word “b’yom” indicates that all Mishkan work was done literally during the day, and not at night.
While this simple answer seems to resolve the second issue regarding the seemingly extra word “b’yom,” we are left to understand the focus on the first day of Nissan, and whether it is the first day Moshe assembled the Mishkan, or, in fact, the eighth day.