The day of the wedding of the Rebbe Shlit"a family is a special day.
Across the world his Chassidim refrain from saying "Tachnun" the whole week of the Chasuna.
However recently the Rebbe Shlit"a has decided that only in the Shul where the Choson Davens should they refrain from saying Tacnun.
Reason: bla"h there are so many family Simchas that if they were to continue with the original custom, Tachnun would hardly be said. In order to preserve the Tefilas Tachnun he decided that there should be a change.