A fantastic speech at J Street, in a day that started with starting the handicapped Boston Marathon [View all]
then seeing the Red Sox.
Here is a link to the J Street speech - http://www.state.gov/secretary/remarks/2016/04/255951.htm (Here is a link to the video - http://jstreet.org/2016-assembly-stream Kerry is on starting at 1 hr 10 min. Biden is at 1 hr 41 min. - An interesting young MK Stav Shaffir speaks earlier at about 29 minutes in.)
Earlier in the day: 
Then before heading to DC for J Street, he went to see the Red Sox. https://www.bostonglobe.com/lifestyle/names/2016/04/18/john-kerry-takes-red-sox-game/eyatStaLPPeX9uXIfnTfZO/story.html
Today, he is in NYC and will be meeting with Zarif.
I would guess that Kerry, who had not until, I think yesterday, been on the schedule for J Street really wanted to thank them for their consistent support of him on the Iran deal and his negotiating for a two state solution -- in addition, the 3 honored ambassadors have long supported him and - at least two seem to be long time friends of his.