Layfeyette's Grave, July 4, 2018. Photo by Deborah Goldfein
General Lafayette's grave lies inside a private cemetery within a church compound that is not easily noticed from the street. Walk up to 35 rue Picpus (not the same as Boulevard Picpus), ring the bell and ask to visit, or if the door is open, find the caretaker. There is a small fee to enter; then you can walk past the church to the burial site of General Layfayette.
There is an invitation-only ceremony every July 4th to place flower tributes and fresh flags on Lafayette's grave, but this is usually done in the morning. The grave is tends to be open to the public in the later afternoon on July 4th.
There are two sources of visiting information below. Because the site is private, less frequently visited (than national monuments) and dependent on the caretaker being there, opening times can be erratic. Best bet is to go Wednesday through Sunday, arriving between 3:00 pm and 4:00 pm. It's well worth the effort!
Meier, A. [Atlas Obscura]. (2013). A Tale of Two Revolutions at the Grave of the Marquis de Lafayette. Retrieved from https://www.atlasobscura.com/articles/a-tale-of-two-revolutions-in-paris
Paris: Official Website of the Convention and Visitors Bureau. Cimetière de Picpus. Retrieved from https://en.parisinfo.com/paris-museum-monument/71410/Cimetiere-de-Picpus
Sloper, S. [Paris Cemeteries]. (2016-2018). Picpus. Retrieved from http://www.pariscemeteries.com/picpus-1#_=_