They are due September 8th.
All the info is at https://friendsofdavidgilbert.org/parole-letter-guidelines/

Last week, on his final day in office, Gov. Andrew Cuomo commuted David Gilbert’s sentence, making him eligible for parole. Since then, a parole hearing for David has been scheduled for later this month. The Parole Board needs to hear that there is widespread support for his immediate release.

Once again David needs your help. Personal letters to the Parole Board are the best way to show your interest in this case. Time is short. We have only one week to collect letters for the Parole Board members to grant David parole at this hearing. Letters must be submitted by 6pm on September 8th.

If you wrote a letter supporting clemency for David, thank you! Please also consider reworking the letter to urge the Parole Board to grant him parole at this upcoming hearing. Every letter helps!

See below for guidelines for the letters. Note that a hard copy of the letter must be mailed to David’s counsellor and a digital copy should be submitted to https://friendsofdavidgilbert.org/parole/