You may know that the planets move in a clockwork fashion. It is in fact God's clock. And He used it to foretell the birth of the Messiah. Only three wise men got the memo... but that was enough. This is what the wise men had to work with...
Genesis 49 8“Judah, you are he whom your brothers shall praise;
Your hand shall be on the neck of your enemies;
Your father’s children shall bow down before you.
9 Judah is a lion’s whelp;
From the prey, my son, you have gone up.
He bows down, he lies down as a lion;
And as a lion, who shall rouse him?
10 The scepter shall not depart from Judah,
Nor a lawgiver from between his feet,
Until Shiloh comes;
And to Him shall be the obedience of the people.
11 Binding his donkey to the vine,
And his donkey’s colt to the choice vine,
He washed his garments in wine,
And his clothes in the blood of grapes.
12 His eyes are darker than wine,
And his teeth whiter than milk.
The wise men understood that this part of the prophecy was an astronomical mark in time... when the king planet crowned the king star in the king constellation. This took place from autumn 3 BC when Jupiter began its triple conjunction with Regulus in Leo... before coming in close conjunction with Venus on June 17 2 BC as a brilliant evening star in the western sky.
So we know that the wise men were in Jerusalem on the way to Bethlehem on December 25 2 BC because it was a moonless night where Jupiter was above Bethlehem as seen from Jerusalem. And the reason king Herod killed the male children in Bethlehem from two years old and under... was because the wise men that winter over a year later told him about Jupiter's first conjunction with Regulus in autumn 3 BC.