'Solemn' in the Bible

“I will also put an end to all her rejoicing,Her feasts, her New Moons, her Sabbaths,And all her festivals.

What will you do on the day of the appointed festivalAnd on the day of the feast of the Lord [when you are in exile]?

But I have been the Lord your God since [you became a nation in] the land of Egypt;I will make you live in tents again,As in the days of the appointed and solemn festival.