The forty-day anniversary is a Pharaonic custom
Question

                What is the origin of the forty-day anniversary? Is there any evidence that eulogizing is acceptable in Islam?
Praise be to Allah.Firstly: 
          the origin of this is a Pharaonic custom which was practised by the 
          Pharaohs before Islam, then it spread from them to other nations. It 
          is a reprehensible innovation (bid’ah) which has no basis in Islam, 
          and it is be rejected as the Prophet  
          (peace and blessings of Allaah be upon him) said: “Whoever innovates 
          anything in this matter of ours (Islam) which is not a part of it, will 
          have it rejected.” (Saheeh – agreed upon).  
Secondly: 
          commemorating and eulogizing the dead in the manner that is done nowadays, 
          when people gather for that purpose and exaggerate in praising the deceased 
          is not permitted. It was narrated by Ahmad and Ibn Maajah, and classed 
          as saheeh by al-Haakim, that ‘Abd-Allaah ibn Abi Awfaa said: “The Messenger 
          of Allaah  (peace and blessings 
          of Allaah be upon him) forbade eulogizing.” (And it is not permitted) 
          because mentioning the attributes of the deceased usually involves boasting 
          and provokes sorrow and grief. But simply speaking well of him when 
          he is mentioned or when his funeral passes by, or mentioning his good 
          deeds and so on, in a manner similar to the way in which the Sahaabah 
          eulogized those who were killed at Uhud and others, is permissible. 
          It was narrated that Anas ibn Maalik (may Allaah be pleased with him) 
          said: “A funeral passed by and they spoke well of the deceased. The 
          Prophet  (peace and blessings of 
          Allaah be upon him) said, ‘He deserved it.’ Then another funeral passed by and they spoke ill of the deceased. The Prophet  
          (peace and blessings of Allaah be upon him) said, ‘He deserved it.’ ‘Umar (may Allaah be pleased with him) said, ‘What did they deserve?’ 
          He  (peace and blessings of Allaah be upon him) said, ‘The first one you spoke well of, and he deserved Paradise, and the second one you spoke ill of, and he deserved Hell. 
          You are the witnesses of Allaah on earth.” (Narrated by al-Bukhaari and Muslim).