Two different kanjis for 'Past'


I am curious as well as confused to the existence of both 去 and 先.

Both these kanjis are used to mean past time, like 去年, 先月 etc. What I wonder
is why different kanjis are used in case of month and year to mean 'last year'
or 'last month'. Is there some historical reason behind the existence of these
two different kanji for the same meaning? I have not yet seen such a case in
my Japanese studies yet.




Most of Japanese people wonder the same thing but seems no one knows the
answer. But I found this [blog](https://whitebear0930.net/archives/18986) most
plausible. It says 昨 means just one previous time (like last month or last
year) and 先 means even before than 昨 (like a few days ago: 先日).




Most of Japanese people wonder the same thing but seems no one knows the
answer. But I found this [blog](https://whitebear0930.net/archives/18986) most
plausible. It says 昨 means just one previous time (like last month or last
year) and 先 means even before than 昨 (like a few days ago: 先日).

