Shavuos 436. Many have the minhag to eat a milchige meal (and milk and honey) on Shavuos. Some eat milchig at night and fleishig at the day meal, while others eat milchig, bentch, and then wash again, and eat fleishig to be yotze Simchas Yom Tov with meat.
Shulchan Aruch w/Mishnah Brurah 494:3
437. If one eats milchig but not “hard cheese” (which requires one to wait six hours before eating meat) one need not bentch and wash again. However, one should wash his /her mouth well and the tablecloth should be changed. (Other poskim require one to bentch).
Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:12