Further to Kame401's opinion on Weddings in Japan, there are a few more important bits of information you should remember when attending a Japanese wedding.
1. The "reception" in Japan is called a "hi-ro-en". This is the occasion where you should present your money. Japanese men traditionally wear black suits with white ties to these occasions (NEVER EVER wear a black tie as this is only worn for funerals) but it is highly unlikely your hosts will mind if you simply wear a smart suit.
2. If you are not immediately related to the family and are simply a work colleague or casual friend, the absolute maximum you should give is 20,000 yen. Any more would be seen as an embarrasment. If a number of you from the same company are attending a friends wedding, one easy way of doing it to throw all the money in together and say it is from all of you.
3. The giving of crisp bank notes is correct but you should never give two notes as this defies that it can be easily "separated". For example, if you deicde to give 20,000 yen, then you should split it into 1 x 10,000yen note and 2 x 5,000 yen notes.
4. You may also be invited to a further party called a "Ni-ji-kai" (lit. a second party). This is a much more informal affairs where most of the participants will be of a similar age and the usual photos/games will be displayed/played.
Thats about it. Apart from the "ni-ji-kai", Japanese weddings are not very relaxed affairs but nevertheless, they are an excellent insight into a very stylized aspect of Japanese culture.
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