JUNE 6th is the closing day, for letters of comment, Brendan.
But don't let that keep anyone from sending a letter. Even June 5th is not "too late".
If you want to make sure it gets there by the deadline date, you can Fax a copy first and mail the letter afterward. (The Fax has to be confirmed with a hard copy).
Thanks to Scooter, for this updated info. See post - #46817 for the details and fax number to Wally Erickson at the DRMS.
Don't take my word for it, Do your own research! Then you will know it's true!