sometimes 先 (saki) means the future, but for ex 先月 means previous month 😳. why? how?
The character 先 means to "move ahead". The character itself is comprised of 之 which represents "footprints" and 儿 which represents a person. I think imagining footprints in the sand gives a good image.
So if you think about it in this sense, it can represent 先月 or 先日 as something that "came before" such as the footprints. However, at the same time, those footprints can represent a course you haven't taken yet. So when someone says "先が楽しみ", you are looking forward in taking that path in the future they haven't explored yet.