40 days

Ta'ziyah (sharing condolences with the bereaved)

Q: My uncle passed away few months ago and I would like to know after burial what needs to be done. Some people have gathering at home for 3 days and some people have gathering for 13 days and some people have gathering for 40 days could you please tell me which group is correct. Also people have told us to have a special dua on 40 days then on 6 months of the death and the last one at 1 year after the death. Can you please tell us what is the correct thing to do?