'Observed' in the Bible

The exiles observed the Passover on the fourteenth of the first month.

And they observed the Feast of Unleavened Bread seven days with joy, for the Lord had caused them to rejoice, and had turned the heart of the king of Assyria toward them to encourage them in the work of the house of God, the God of Israel.

Now I assembled them at the river that runs to Ahava, where we camped for three days; and when I observed the people and the priests, I did not find any Levites there.