Session recap(s)
Every session should get a written recap, an official history of what happened. Few people can recall the events of a session a few days later, much less after weeks or months. So make sure the key moments of each session get written down.
If one of your players takes session notes, then ask them to post those notes somewhere public. Review them before you start prepping for the next session, and make corrections or additions as needed.
You might find it helpful (or necessary) to make a session recap yourself. Maybe your players don't take notes, or their notes are so detailed that the other players will never read them. Maybe they leave out stuff that you think is important.
There's no one right way to do this. Some groups like a detailed, scene-by-scene breakdown. Others prefer a bullet-point of key events, like this: