Udhiyyah or making sacrifice during Eid al Adha and on the days of Tashrik is a commendable act.
The problem arises when one wishes to make udhiyyah for the dead.
The popular ruling according to Imamuna al Shafi’i is that it is not commendable to do so UNLESS the dead wished for it ( making wasiyyat ) before his/her death!
However certain scholars amongst the Shafi’i mazhab were reported to have said that the act is commendable and the thawab will reach the dead.
This was reported by Imam Muhyiddin al Nawawi in his Majmu’, Imam al Hafiz Ibnu Hajar al Haitami in his Tuhfah al Muhtaj and al Khatib al Sharbini in his Mughni al Muhtaj.