Amazing!!! It seems many reviewers missed important details.

Rating: 10/10 (12 October 2019)

I avoided this movie for over a year because of the negative reviews on Amazon and IMDB, although a couple of friends thought it was "pretty good" I figured it was another run of the mill Insidious style horror films...boy was I wrong!
Do yourself a favor, especially if you like attention to detail, and watch this movie!
**SPOILERS AHEAD**
From the start of the movie you begin seeing a strange symbol (Annie and Ellen Leigh's necklace), I noticed immediately that it would be of significance. Pay attention to the funeral attendees, you'll be seeing them later on. Also pay attention to the words Charlie has carved into the walls over her family members beds.
During a grief counseling session Annie discloses that she attended therapy in the past, that she had a brother who hated their mother and committed suicide because Ellen (Leigh) was "trying to put people into him." It's implied that the same happened to Joan's son and grandson, who both died by drowning. I interpreted this to mean Joan's son knew his boy would be a viable host and thus drowned himself and his small son. Annie even admits to her son (Peter) that her mother basically forced her to get pregnant and have him, that she'd kept him away from her as a result. However, when Charlie was born (expected to be a boy) Ellen "doted on her" even insisting on breastfeeding the baby herself, which we see in one of Annie's miniature recreations. This behavior wore down Annie's defenses until Annie finally relinquished and "gave" Charlie to Ellen. This lets us know that Ellen promised King Paimon (a prominent figure in demonology) a suitable (male) host and although Charlie was expected to be a boy (hence the Charles welcome mat in Ellen's box of things), he settled for a girl because she'd already summoned him. This means that Charlie had been Paimon's host from the moment she was born, and so had Peter. This revelation explains a TON, from Charlies physical defects, to her odd obsession with headless "toys" (figurines), to her decapitation of the deceased bird, to the clicking noise she makes.
Charlie's death was also orchestrated long before they even left home for the party. We see this in Paimon's symbol etched into the light post, to the party girl manically chopping nuts next to the chocolate cake poor Charlie will later consume. There were multiple avenues of death awaiting Charlie, just as there were multiple avenues of resurrection awaiting Paimon (note the pamphlet that slips through Annie and Steve's mail slot).
Hereditary also serves as a deep allegory for mental illness:
Ellen was diagnosed with D.I.D (dissociative identity disorder), Annie's brother was a paranoid schizophrenic, which eventually led to his suicide. Annie herself suffers from mania and possible bipolar disorder (the mania is noted in an email we see Steve drafting --it's implied that he was once her therapist). Even Peter's classroom hallucination self-harm episode could be explained away as the beginnings of schizophrenia. All these mental illnesses are manifesting in the next generations, so they've inherited a curse any way you slice it. Thus, Paimon serves to not only give the coven what they seek, but to also deliver Annie and Peter from the prisons of their dwindling minds.
There was so much thought and research put into Hereditary, everything in this film is intricately and beautifully interwoven, which is one of the aspects that make Hereditary so fantastic.