The early Christians met on every first day of the week (1 Cor. 16:2). [Greek: Kata Mia, every first day. Compare Kata Men, every month (Rev. 22:2) and Kata Polis, every city (Acts 15:21)]. In 1 Corinthians 11:20, Paul censures what these Corinthians did "when you meet together" (every first day, 16:2), and explains what they should do, viz. partake of the Lord's Supper (vv. 23ff). It's a "left-handed" argument, but it is clear and strong.
Acts 20:7 says the disciples met upon the first day of the week to break bread. "The" is the definite article, expressive of a stated or fixed day. There is certainly no authority for the Lord's Supper on any other day. Further, here is equal authority for the Lord's Supper on each and every "first day" that rolls around. (Example: "Remember the Sabbath . . ." Ex. 20:8. Which Sabbath Day? The Jews had no difficulty in recognizing that this was authority for every 7th day that came). These are scriptural arguments which I hold to be sound.
Reasonable evidence is found in the fact that all commemorative institutions which God ordained in prior times (Law of Moses) had a fixed and regular time for observance (The Passover, Feast of Tabernacles, Purim, Pentecost, etc.). If the Lord's Supper is not to be observed every first day, there is no way to determine a fixed time (with divine authority) and this vital memorial (1 Cor. 11:23ff) is left to wander aimlessly.
Finally, secular history vindicates our conclusion. For the first three centuries, all the churches broke bread once a week. The weekly communion was prepared in the Greek church until the seventh century. We give one quote for those interested: "But every Lord's Day do ye gather yourselves together, and break bread . . . etc." (from a writing called "The Teaching of the Twelve Apostles" dated about 120 A.D. Taken from Vol. 7, p. 381, Ante-Nicene Fathers).
Why do I observe the Lord's Supper every week? Because of the word of God which makes it clear that this is God's will for His people.
Now, be honest with yourself. Can you give as good a reason for observing this memorial less frequently? And how can you justify the common Thursday night observance?