It's kind of an oxymoron, really.
I can find no scripture which connects worship exclusively with solemnity. There is grieving worship - the whole of Lamentations, for example; or the 137th Psalm. In them, Israel mourned the sin that led to their expatriation. The Lord's admonition to keep silent before Him in His temple (which we often see cut from context and cut into wood plaques above sanctuary doors) was a command to repent from idolatry to "gods" of stone and wood in Habbakuk 2. There is mourning for people who are dead or at least thought dead. There is James' advice to quarrelsome brothers and sisters in chapter 4 to mourn in their penitence.
(Maybe some of our brothers and sisters who cause dissension and division by forbidding worship which entertains God and man should mourn ... and repent. - Then get over it, and experience some real joy!)
While there are these few examples of mourning and worship connected in scripture, what you will find throughout the Old and New Testaments are hundreds of examples of worship accompanied by joy.
From the poet's exultation in Psalm 100 to the disciples' recognition of the risen Lord in Luke 24:52, worship, praise and joy go together.
If you surround the Lord's table and do nothing but mourn His death, week after week after week, know this: HE IS RISEN! (Matthew 28:7; Mark 16:6; Luke 24:6 - why do you think most Christians celebrate it on the first day of the week?!?)
If you gather time after time after time to share the bread and the cup while only mourning your sins, know this: THEY ARE FORGIVEN! (Psalm 32:1; Acts 2:38; >Ephesians 1:7; 1 John 2:12)
If you sing songs of joy and gratitude Sunday after Sunday after Sunday with only muttering gravity in your voice and duty in your face, know this: THE JOY OF THE LORD IS YOUR STRENGTH! (Psalm 28:7; Nehemiah 8:10)
Good people, if joyless fear and dread and silence is always and only to be the hallmark of worship acceptable to God, why is that Jesus, "full of joy through the Holy Spirit, said, 'I praise you, Father, Lord of heaven and earth ...'"? (Luke 10:21) Why then did His followers loudly and joyfully praise God for His miracles? (Luke 19:37)
If we are still to live and worship only in fear, why does the Hebrews writer go to such pains to distinguish the old covenant's mountain of fear from the new covenant's mountain of joyful angels? (Hebrews 12:18-24)
If our obedience to God in worship and life is paramount, why do we ignore 1 Thessalonians 5:16 - "Be joyful always"? Is the instruction to "entertain strangers" (Hebrews 13:2) not to be obeyed in the context of gathered worship?
You can't even find the word "solemn" in the New Testament, except as a description of an oath to assassinate Paul (Acts 23:14)! The New Testament is a testament of gospel; of good news; of great joy that shall be to all people!
Doesn't that bring you joy? Doesn't that bring enjoyment, knowing it? Shouldn't our worship to God reflect our enjoyment of His blessing, and entertain Him with our praise?
I recall a story told in a church I used to attend, of a brother who once asked a sour-faced elder if he was a happy person. "I suppose so," was the grudging response. So he answered, "Then why don't you let your face know it?"