'Solemn' in the Bible

Behold, I build an house to the name of the LORD my God, to dedicate it to him, and to burn before him sweet incense, and for the continual shewbread, and for the burnt offerings morning and evening, on the sabbaths, and on the new moons, and on the solemn feasts of the LORD our God. This is an ordinance for ever to Israel.

And in the eighth day they made a solemn assembly: for they kept the dedication of the altar seven days, and the feast seven days.

Even after a certain rate every day, offering according to the commandment of Moses, on the sabbaths, and on the new moons, and on the solemn feasts, three times in the year, even in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles.