As in English, the first month of the Spanish year is named after Janus, the Greek double-faced god, who looks behind at the past year and forward toward the new. Janus was also the god of thresholds, liminal spaces and doors, likely accounting for all the back and forth in the narrator’s words and the reference to the “key.”
Some more information from wikipedia, on Janus, may also clarify some of the questions and comments above:
While the fundamental nature of Janus is debated, in most modern scholars' view the god's functions may be seen as being organized around a single principle: presiding over all beginnings and transitions, whether abstract or concrete, sacred or profane. Interpretations concerning the god's fundamental nature either limit it to this general function or emphasize a concrete or particular aspect of it (identifying him with light the sun, the moon, time, movement, the year, doorways, bridges etc.) or else see in the god a sort of cosmological principle, interpreting him as a uranic deity.