A Grammatical Dictionary of Botanical Latin

January (month):

Noun: Januarius,-ii (s.m.II), abl. sg. Januario, also Ianuarius,-ii (s.m.II), abl. sg. Ianuario; often associated with dies, gen. sg. diei (s.m.I), abl. sg. die, ‘day.’ [Januarii = gen.sg.; Januario = abl.sg.];

- in die primo Januarii on the first day of January.

- die 2 Jamuarii, on the second day of January.

- diebus primis Januarii, in the first days of January.

- alga die 20 Januarii lecta, alga on day 20 of January collected.

- Januario, in January.

- exeunte Januario, at the ending of January, January going out, departing.

- semel, ineunte Januario, in pyris sub dio projectis (S&A), one time, January coming in [i.e. at the beginning of January], on a pear tree out-stretched under the open sky.

Adjective: januarius,-a,-um (adj.A) (ianuarius,-a,-um (adj.A)), of or belonging to Janus; modifying nouns, but usually modifying the noun mensis (s.m.II), abl. sg. mense, ‘month’ or dies, gen. sg. diei (s.m.V);

- in die primo Januario, on the first day of January.

- die 2 Januario, on the second day of January.

- diebus primis Januariis, in the first days of January.

- [fungi] tempus servat Cortinariis succulentis plerisque solemne, vigens nimirum a medio Januario ad medium usque Martium (S&A), ordinary weather preserves Cortinarius [i.e. species], and many succulent [species] flourishing of course from the middle of January to mid March.

NOTE the specific "time when" or "within which" something happens is expressed by the ablative without a preposition.