Christian worship is fundamentally communal. Our spirituality happens in community, as the Church. In fact the highlight of Christian worship, at least traditionally, is oftentimes called Communion; the Eucharist. Because our lives as Christians are intimately bound and interwoven with one another, because corporately we are the Body of Jesus Christ in this world. That means gathering together and "doing" liturgy: prayers, hymns, confession and absolution, making peace and being united in like mind and in like faith through our communal and corporate offering of worship to God.
We can do this any day, and arguably many, if not most, churches offer worship throughout the week. However from nearly the beginning Christians have gathered weekly on the first day of the week for this express purpose.
Most religions have some form of rhythm, and that only makes sense since our lives are rhythmic. Day and night, weeks, months, seasons and years. There are natural rhythms in life. People have, throughout history, found ways to sacralize time. Christians do the same, there is the weekly rhythm of the first day of the week being gathered together to come together in worship, to receive Word and Sacrament. Christianity also sacralizes time through the Liturgical Year, with seasons, feast days and fasts. So, for example, yes we can and should celebrate the Mystery of the Incarnation throughout the year, but we specifically set aside certain time in the year for that purpose that becomes part of an organic rhythm in the Liturgical Year--we call it Christmas.
Since Christianity isn't centered upon individual spirituality, but the communal sharing of life together in Christ we do things together, as a community, as the Church. Not that individual spirituality is unimportant, only that my individual spirituality arises principally out from my corporate life in the Body of Christ. I could read Scripture all the time, but it's not the same as receiving the Word as spoken when we are gathered together. And one cannot celebrate the Eucharist alone, by its very nature it is a communal moment where the Church celebrates the deepest mystery of our unity together as the Body of Christ--that is, we are the Body of Christ because we receive the Body of Christ.
-CryptoLutheran