It has been 11 years since we've had a year with this configuration which places Rosh Chodesh Sivan on Sunday, thus pushing aside the regular haftarah for Bemidbar in favour of the special haftaras machar chodesh. There is an intriguing connection between the haftarah and Shavuos, as detailed by R’ Elie Wolf.
The haftarah is from Shmuel I perek 20. Towards the end, Sha'ul gets rather annoyed at his son Yonasan for siding with his friend, David. In the midst of his outburst, he exclaims, (pasuk 30) "Son of a rebellious woman! Do I not know that you choose ben Yishay to your own shame and the shame of your mother's nakedness!?" What is the reason for such an outburst and what does Yonasan's mother have to do with anything?
Rashi on this pasuk tells the story of how Sha'ul met his wife. After most of the tribe of Binyamin were wiped out following the gruesome episode of pilegesh b'Giv'ah at the end of Shoftim, the tribe was in danger of extinction. They were told to go out to the vineyards and watch as the daughters of Shiloh come out and dance and they were to pick wives from them. Sha'ul was embarrassed and did not partake in this exercise until finally, his eventual wife uncharacteristically ran after him and, well, the rest is history.
The gemara (Yevamos 76b) tells the story of how the validity of David's lineage was questioned due to the fact that he descended from Rus the Moabite, a passage with obvious implications to Shavuos when we read Megillas Rus. Avner maintains that the prohibition of a Moabite (or Amonite) to marry into B'nei Yisrael (Devarim 23:4) applies only to males (Moavi v'lo Moavis) and therefore Rus was allowed to marry Bo'az and David's lineage is clean. The rationale he suggests is that the reason given for the prohibition is that they did not come out and greet B'nei Yisrael with food and bread. This can only be a claim on the men for it is not the way of the woman to go out and greet. Do'eg retorts that they should have brought out the men to greet the men and the women to greet the women, to which Avner seemingly has no response. The gemara later concludes that the rebuttal to Do'eg's claim is that even still, the pasuk says, (Tehillim 45:14) "kol kevudah bas melech penimah," the honour of the princess is to dwell within. Even to greet the women, it would not have been right to make the women come out. Aruch LaNer suggests that the reason why Avner neglected to offer this rebuttal is because he did not want to insult Sha'ul haMelech for the manner in which his wife seized him was clearly a breach of this maxim. Therefore, he chose to remain silent.
Chid"a and Chasam Sofer suggest that this is the explanation of Sha'ul's rebuke of Yonasan. If Yonasan is choosing to side with David, he is affirming the legitimacy of David's kingship which is based on the adage "kol kevudah bas melech penimah." By doing so, he is effectively shaming his own mother for the way she seized Sha'ul.