Is it permissible to do Ath-Thakeerah, which is to prepare lunch or dinner on order to remember the dead, or to do so when someone sees him in a dream?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

This memorial is an innovation. 'Aa'ishah narrated that the Prophet said: "Whoever introduces into this matter of ours (religion) that which is not a part of it, it will be rejected.” [Muslim] What the family of the dead should do instead is to supplicate Allaah to forgive him and have mercy on him, give charity on his behalf and perform Hajj or ‘Umrah on his behalf if he had not performed them when he was alive. If a person sees the dead in a dream, he should ask the scholars who interpret dreams to interpret it for him, and then he should act according to their instructions. 

Allaah Knows best.