Alex's tweet before his attempted suicide, "Ask for me tomorrow, and you shall find me a grave man", is a line spoken by Mercutio before his death in William Shakespeare's "Romeo and Juliet".
At dinner, Sarah (Aubrey Plaza) mentions that it feels like a big 80s movie around the table. This movie very closely resembles an 80s movie called The Big Chill, where a group of college friends reunite after a funeral of one of their friends, also named Alex.
Another strong similarity to "The Big Chill" is that one character brings his much younger girlfriend with him (Meg Tilly in the older film; Jane Levy in this one), and the rest of the group doesn't know her at all.
In Alex's house, on top of his refrigerator, a model of the infamous ill-fated ocean liner RMS Titanic can be seen.