Weather

This will depend on the season and where you are going so I will share tips for summer and winter in Tokyo.

Summer - June to Mid October, around 30 deg C

Winter - Mid November to March, lowest is around 0 or -2 deg C

Culture

Although Shintoism and Buddhism influence a lot of Japanese culture, Japan is a secular country. There are no religious rules regulating clothing, however, you may notice that Japanese people dress relatively conservatively (as in they do not over-expose skin).

What you wear is obviously completely up to you, but here are some common practices outside of Japan I usually avoid when I go home for my own comfort.