The second Haftarah of the Three Weeks, which is the second chapter of Yirmiyahu, always falls out on Parshat Mas’ei.
Linear annotated translation of the Haftarah of Mas’ei
While the Parsha is about how the Jewish People followed G-d in the desert for forty years, the Haftarah keeps repeating how the Jewish People stopped following G-d’s ways and compares their path to a drunken camel in heat.
But on a deeper level, there is a connection between one of the commandments in Mas’ei and one of the sins for which Yirmiyahu condemns the Jewish People:
Chazak, Chazak, ve’Nitchazek!