She edited and compiled the Card Index of New Genera, Species, and Varieties of American Plants, a quarterly publication that was considered "indispensable" to botanists.

[1] On January 1, 1893, Day moved to Cambridge, Massachusetts, where she was appointed librarian of the Gray Herbarium of Harvard University.

Her work involved checking bibliographical references, proofreading manuscripts, preparing indexes and statistics on the library and herbarium collections.

"[4] Day's first major assignment was the verification of about 5,000 bibliographical references in a collection of manuscripts by Asa Gray and Sereno Watson, which were being prepared for a posthumous publication by the curator William Coolidge Lane.

Day prepared the publication between her regular duties as a librarian, indexing over 130 scientific serials, including foreign language monographs.