According to the opinion of the Ba’al Halachos Gedolos (BH”G), the mitzvah of sefira is to count 'sheva shabbasos temimos', seven complete weeks, and therefore, if one misses counting sefira on any single night, the entire mitzvah is lost. Tosfos (Megillah 20b) disagrees, and holds even if one misses one night, one may count the remainder of the nights. Many explain (based on the Rosh) that the issue at stake is whether sefiras ha’omer is one mitzvah encompassing 49 days, in which missing a day renders the entire unit void, or whether counting each day is a separate mitzvah.
Even though on a regular Shabbos or Yom Tov one may accept Shabbos/Yom Tov early, the popular minhag follows the Taz who writes that on leil Shavuos which brings the omer count to a close one is not permitted to daven ma’ariv before tzais hakochavim, actual nightfall. The reason behind this chumra is in order to make sure the final day of sefira is “complete”, i.e. the criteria of “temimos” is met.
I am sure there is an easy answer I am missing, but I do not understand this fully. If sefira is one unit of 49 days, I understand the logic of seeing the unit to completion on the last day by waiting until nightfall. But if each day is a seperate mitzva, then shouldn't the criteria of 'temimos' be applied to eacy day (rather than to the unit as a whole) individually? If so, on any given Friday night (or weekday night, although less common a practice) during the sefira period one should not be permitted to daven ma’ariv early because in doing so one is subtracting time from the individual day and rendering it incomplete. I have never heard anyone suggest such an idea. Why not?