January 26, 1968
I guess it's impossible to be absolutely certain but surely if we have multiple posters indicating that shows took place at this venue on the 26th and 27th. The lack of evidence that shows took place on the 22nd and 23rd is, to me, a compelling case that there were only two shows at Eagles Auditorium, Seattle at this time.
The fact that we have only two complete-ish setlists for the four nights is, in my opinion, more evidence that only two shows took place.
I agree with many users on LMA that the Spanish Jam circulating as the only fragment we have from the 27th is the remainder of the Spanish Jam that is cut on tapes purporting to be the 23rd.
Road Trips Vol. 2, No. 2 has a BIODTL, Hurts Me Too, Viola Lee Blues, Dark Star> China Cat> The Eleven and Lovelight purporting to be from the 23rd. If one puts together these tracks from the other circulating "23rd" stuff, one gets a fairly typical early 68 setlist that, I believe, was actually played on the 27th.
The "22nd" tape is, in my opinoin, the 26th.
In my opinion, it makes a lot more sense that there were two shows, as we have music identified from only two consecutive nights in Seattle and only have evidence of two shows taking place from posters, advertisements, etc. I think the dates have been mixed up at some point (e.g. mis-recorded by the band when preparing Anthem, mis-read, etc) and the 26th and 27th shows have been mistakenly believed to have taken place on the 22nd and 23rd.
http://jgmf.blogspot.com/2010/05/gd-at-eagles-auditorium-seattle-wa.html lays out some of the evidence around these datings.