.ל"א The reading for מחר חדש

חל להיות באחד בשבת מאתמול מפטירין ויאמר לו יהונתן מחר חדש

Yehonasan tells David that tomorrow is Rosh Chodesh and there will be a seudah. (For a discussion on how exactly Yehonasan knew it would be Rosh Chodesh since it was based on testimony, see AstroTorah.) They devise a whole plan to confirm whether Shaul still bears mortal enmity towards David. R' Shimon Schwab, in Maayan Beis HaShoeiva, is bothered, is it only because of the fact that Yehonasan mentions the words machar chodesh that we read this haftara? Surely there is a greater reason to push aside the regular haftara in favour of this one. 

Rav Schwab explains that Yehonasan's intentions in telling David not to escape until after the seudah of Rosh Chodesh were because Rosh Chodesh, as the moon enters a new cycle around the Earth, symbolizes a time of renewal, on opportunity for repentance and atonement. Yehonasan was telling David to wait and see if Rosh Chodesh will possibly have this positive effect on Shaul and he will change his mind. Therefore, we read this haftara to impress upon the masses this important aspect of Rosh Chodesh. Additionally, Rosh Chodesh is a time destined for geula which is why we recite in Mussaf, Mizbeach chadash b'Tzion tachin. And if Moshiach doesn't come, we are left wondering "why has ben Yishai not come, not yesterday and not today" (20:27, apasuk in the haftara.) The Navi remarks, vayipaked mekom David, David's place was vacant, an allusion to the fact that we, too, are lacking the presence of ben Yishai. And just as it was gratuitous hatred that caused David's absence from the party, it is exactly that that causes Moshiach to be absent today.