Shavuos 438. One should be careful not use leftover challah from a milchig meal at a fleishig meal. (This is applicable all year as well). Rather, one should use new challahs for each meal. This also fulfills the minhag of rememberance of the Shtei Halachem (Two Loaves) brought on Shavuos in the Beis Hamikdash. Shulchan Aruch w/Mishnah Brurah 494:3

439. From Rosh Chodesh Sivan until the 8th of Sivan (Isru Chag) we do not say Tachnun. Some do not say Tachnun until the 12th of Sivan, and in the Diaspora until the 13th of Sivan.
Shulchan Aruch w/Mishnah Brurah 494:3, Piskei Tshuvos 494:9