Halakhah do Psalmów 137:6

Should any of these four fast days occur during the bridegroom's seven days of wedding feast, although these days (of the wedding feast) are considered to him as a Yom Tov, nevertheless, he must fast.¹This law also applies when any of these fast days is postponed, as stated in paragraph 6. A groom must fast even on a postponed fast day. (Mishnah Berurah 559:9,35) Since his Yom Tov is only a personal celebration, the public mourning and fasting overrides it, and also, because it is written, "… if I fail to elevate Jerusalem above my foremost joy" (Psalms 137:6).