Tertio-millennial whatto?

The Santa Fe Tertio-Millennial Celebration and Exhibition 0f 1883 was not the most roaring success in terms of attracting visitors. Perhaps it was because of the name. Also, 1550-1883? Hmm. It ran from July 2 to August 3, "the third day of the 333rd year." Elsewhere it celebrates "a third of a thousand years."

Library of Congress Printed Ephemera Collection; Portfolio 101, Folder 3a.