Herod died at the lunar eclipse of March 13, 4 B.C. according to Josephus (Antiquities 17.6.4). This is a historical fact not disputed by any serious historian.

The reason he commanded the children in Bethlehem 2 years and younger to be slain is because the Magi had come to Jerusalem 2 years prior, the year Christ was born, in 6 BC. Jesus of Nazareth was born 2 years before the death of Herod on the night of December 17th-18th at the Jupiter stationary transit in Aries.

[link to penelope.uchicago.edu]

This eclipse of the moon, (which is the only eclipse of either of the luminaries mentioned by our Josephus in any of his writings;) is of the greatest consequence for the determination of the time for the death of Herod and Antipater; and for the birth, and entire Chronology of Jesus Christ. It happened March 13th, in the year of the Julian period 4710, and the 4th year before the Christian Æra. See its calculation, by the rules of Astronomy, at the end of my Astronomical Lectures, edit. Lat. page 451, 452. See also my VI Dissertations, pag. 336, 339. And Note that in the IIId and IVth of those VI Dissertations, I have largely shewed, what vast use may be made of eclipses, both of the sun and moon, not only in Chronology, but Christianity also; and even in the support of scripture prophecies themselves, as to the grand periods of the four monarchies, and the evident interposition of Divine Providence in the changing and putting an end to those monarchies, pag. 133–267.