Atemoztli.

24. Dediziēbre. 4. Tecpatl, atemoztli = 24 December. Four flint, atemoztli. (central Mexico, early seventeenth century)
Codex Chimalpahin: Society and Politics in Mexico Tenochtitlan, Tlatelolco, Culhuacan, and Other Nahuatl Altepetl in Central Mexico; The Nahuatl and Spanish Annals and Accounts Collected and Recorded by don Domingo de San Antón Muñón Chimalpahin Quauhtlehuanitzin, eds. and transl. Arthur J. O. Anderson and Susan Schroeder (Norman: University of Oklahoma Press, 1997), vol. 2, 126–127.

The falling of water from the sky (rain) was celebrated in the sixteenth festival of the year, devoted to Atemoztli. The ceremonies associated with this festival were dedicated to Tlaloc, and their purpose was to persuade the rain to fall from the sky.
Bernardo Ortiz de Montellano, "Las hierbas de Tláloc," Estudios de cultura náhuatl 14 (1980), 287–314, see p. 292.

The Codex Magliabecchi also attests that the reference is to the descent of water from the sky (rain).

One can also see the iconography of the month name from the Codex Telleriano-Remensis in our Visual Lexicon of Aztec Hieroglyphs.