I thought it was 2 days later at Sybrina's deposition 3/15/13
WITH RESPECT TO THE STATE’S RESPONSE TO DEFENDANT’S MOTION FOR SANCTIONS AGAINST STATE ATTORNEY’S OFFICE FOR DISCOVERY VIOLATIONS ON 29 MARCH 2013.
15 March 2013 is what the prosecution's summary of production says (13th supplemental production, 27th of March 2013), noting that a letter from Witness 8 to Sybrina was previously provided.
O'Mara told the press that he received it the day of Witness 8's deposition. I'll go with O'Mara's recollection, although he might be doing the prosecution a favor by assigning that particular earlier date. I give O'Mara's remark credence becuase he would easily remeber receiving that letter on the same day he is scheduled to depose that witness.
George Zimmerman case: State reveals new letter from key witness in Trayvon Martin shooting - By Jeff Weiner, Orlando Sentinel, 3:08 p.m. EDT, March 29, 2013
In an interview with the Orlando Sentinel on Friday, defense attorney Mark O'Mara said he received the new letter March 13, the day he deposed Witness 8 in South Florida."I'm glad I have it now. I don't know when (the state) received it," O'Mara said, adding prosecutors would not say when they did.