In theory, I don't see why Christians can't gather to worship at any time on any day of the week. In certain contexts, where Sunday is a normal working day, it may be necessary to meet regularly on a Saturday. However, Sunday morning has the advantage that it is resurrection morning, and that sets the tone. Every Sunday is a little Easter, and therefore every Sunday gathering to worship is a celebration. A few observations:
1. It seems appropriate that we have a Sunday Celebration rather than a Sunday Service. The former reminds us that our primary purpose is to come together to rejoice in what God has done for us in Christ; the latter sounds like we are coming together to do something for God. In fact, it is our celebrating on Sunday that will empower our service Monday through Saturday.
2. The appropriate tone of Christian worship is always joy. Even when the subject matter of the sermon, or the theme of the day, is something dark - perhaps mourning over sin, or the suffering of death - it occurs within the bracket of joy because it happens on Sunday morning. We live this side of the resurrection, and we know a joy that puts all sadness in its place. The structure and content of our meetings together should reflect this.
3. Nothing can be allowed to stand in the way of joy and its expression, least of all a misplaced concern for reverence. Yes, our approach to God must be in awe and reverence, but too often this is a front under which under which worship is turned into a sombre exercise. Does our worship encourage and display joy?