Will it be a good option to get foreign currency way before planing a trip like a month ago, just because of a favorable deal and good exchange rate?

You will never know, when and what a best deal is.

If you buy now, the rate may change much better (or worst) in a month

Also to be frank, unless you are changing 1000's of dollars, (in which case you don't really you use credit cards) what little difference it makes, it does not really matter