*"Kouhai" means one's younger friends, mates, or colleagues in Japanese.
It's one of the Japanese customs to call younger people as "Kouhai".
Off course, we always call their name when we meet them.
The opposite word of "Kouhai" is "Senpai" which means senior, elder people for me.
I'm going to join his wedding party from 5:30 PM.
It's not wedding ceremony, but just a party.
There're some differences between wedding ceremony and wedding party.
Wedding ceremony is formal event in Japan, so we dress up formal wear and new married couples invite our family, close relatives, friends, colleagues and so on.
*Especially, I think it's an interesting custom to present some money for a new married couple instead of gifts. We generally present "Thirty thousand Yen" in our friend's ceremony, it's not cheap for younger people.
Wedding party is informal event after wedding ceremony, so new married couples invite friends and colleagues. We generally enjoy doing somethings - talking with new married couple and friends, drinking alcoholic drinks, playng quize and bingo, watching videos for new couples.
If we'd like to invite many friends and colleagues, we hold wedding party besides wedding ceremony, because capacity of ceremony is limited.
My purpose of joining my Kouhai's wedding party is celebrating new married couple, but it's not only one.
Meeting with my friends that I've never seen long time. It's also important purpose.
I'm looking forward to seeing new married couple - Kouhai and his wife - and my friends in my universityhood.