A Bris that was pushed off cannot override Shabbos or Yom Tov. The reason (Shabbos 133a) is that there is both and Aseh and a Lo Saaseh regarding performing forbidden work on Yom Tov, while Milah is only an Aseh.(Milah at its proper time has a special Pasuk- "Bayom" meaning even on Shabbos or Yom Tov)

Another reason is that the Pasuk regarding Yom Tov work says "Hu Levado Ye'aseh Lachem"- "this itself will be done for you." The first word Hu is superfluous and excludes any preparations which were not done previously and the next word "Levado" excludes a Bris which was Nidcheh that does not override Yom Tov.

Yoel Domb