intercalation, illegitimate throne
閏 centers on inserting an extra period into a calendar to keep it aligned: intercalation. The rare sense of an illegitimate throne extends this idea of something inserted where it does not naturally belong.
閏 combines 門 (gate) with 王 (king). The traditional explanation ties it to the ancient practice of the king staying inside the gate during the intercalary month, though the exact historical development is uncertain.
Picture a king 王 standing inside the palace gate 門, inserting an extra month into the calendar while he stays indoors. That inserted month is intercalation.
For ジュン, imagine the king inserting an extra month in June: June -> ジュン, and the king stays inside the gate to adjust the calendar.