What is "l'Mo'ed David"?
Rashi, Targum Yonasan: It is the time that David set.
David had said "Ad ha'Erev" (verse 5). Why did Yehonason go in the morning?
Radak: There was no need to wait until evening; he already saw his father's intent. 1
Malbim: [Yehonason] said "ka'Es Machar ha'Shelishis 2 " (verse 12). He went at the beginning of the day.
It would arouse great suspicion if he went at night, especially right after Rosh Chodesh, when there is almost no moonlight. Presumably, "Erev" means late afternoon. (PF)
Perhaps he changed from David's suggestion to go at night, for he wanted to give a Siman via the arrows. Even with a torch, it would be very hard to find them! Also refer to 20:35:2:1*. (PF)