This is the 4069th most frequent Spanish word.
diciembre
December.
Here, 'diciembre' is used as a proper noun referring to the last month of the Gregorian calendar, showing possession with 'es'.
Diciembre es mi mes favorito del año.
December is my favorite month of the year.
In this sentence, 'diciembre' is used as a temporal reference for when an event (cold weather) typically occurs.
En diciembre, normalmente hace frío en el hemisferio norte.
In December, it is usually cold in the northern hemisphere.
'Diciembre' here is in the context of a specific date, marking the start of an event.