why fin janvier?

Question

Why "fin janvier" but not "a la fin de janvier"

Accepted Answer

Hi Carl, 
This is an excellent question and got me researching ...
It appears to be simply for reasons of the principle of linguistic economy and simplification.
Over time, this shorter form has become standard in everyday speech as language evolves and people take shortcuts that become accepted.
As Chris says, to denote a specific time of the year, we will use the shorter version to speak about the beginning, the middle, or the end of a month.
This trend towards abbreviation and simplification is a natural part of language evolution, often driven by the need for more efficient communication in daily life.
French is a very verbose language and it helps to have more simplicity whenever it is offered to you.
Bonne Continuation !

Answer

When referring to parts of months, "début", "mi-" and "fin" are used without articles or prepositions. Used this way, they refer to a less specific period.

fin janvier -- late January
à la fin de janvier -- at the end of January (i.e., the very last few days of January)