Since Lehndorff also remarks on Fritz' birthday (January 24th) a couple of times (whether or not Fritz himself was present, EC had to throw a reception for it), I know that date now, too, but the Heinrich date is really unmistakable, what with all the sparkly hearts in Lehndorff's calendar. ("I am convinced that as many have good wishes for the prince as those who wear clothing on their skin. For to know him is to love him. I seek out my dear H., embrace him tenderly and await with impatience the next opportunity to be alone with him. (...) I feel something for him I haven't felt for anyone else. Sometimes I wish he was poor, so I could give him anything I have; at other times, I would be ready to do the most humble service if only that meant I could be always with him.")

You absolutely need a driver, because while there's a great and very fast railway connection to Berlin from practically everywhere else in Germany, Wust, Rheinsberg and Küstrin are a public transport nightmare as far as I can see. Now if you should, in theory, come during the summer months, and warn me ahead of time, we could, in theory, work something out...

Heinrich's birthday up to Fritz' birthday in 1786 was the last time he saw Fritz alive since he came to Potsdam for the occasion(s).