Konbanwa is good evening in Japanese. You would usually start saying Konbanwa after dinner rather than konnichiwa. It is the most common way to say good evening.
You pronounce it like kohn-bahn-wah.
At night you would usually say Oyasumi nasai. This is saying goodbye at night. Oyasumi nasai is more often used when parting at night rather than saying hello at night. When you are around friends, classmates, close family members, or anyone else with whom you can speak to familiarly you could shorten it to Oyasumi.
You pronounce it like Oh-ya-soo-mee OR Oh-ya-soo-mee na-sigh.
