Constitution Day is September 17th.
The Senate of the United States shall be composed of two Senators from each State. . .and each Senator shall have one vote. (U.S. Constitution, Article I, section 3)
During the stifling hot summer of 1787 the framers of the Constitution were meeting at Independence Hall in Philadelphia. Though delegates agreed to many details, the issue of representation threatened to destroy the work of the convention.
Delegates from large states ascribed to the nationalist principle, believing that because their states contributed proportionally more to the nation's financial and defensive resources, they should enjoy proportionally greater representation in the Senate as well as in the House. Small-state delegates favored the federalist principle and demanded that all states be equally represented in both houses.
In order to resolve these differences, Roger Sherman and Oliver Ellsworth constructed a Great Compromise. Ellsworth explained that equal state representation was imperative in a Union "partly national, partly federal." Sherman proposed a specific agreement for a dual system of representation. In the House of Representatives, each state would be assigned a number of seats based on its population. In the Senate, all states would have the same number of seats, and thus in the Senate each state would be equal. Due in part to this decision, the Senate has remained a more deliberative body where minority views have ample opportunities to be heard.
On July 16th, the convention adopted the Connecticut Compromise by a margin of a single vote. Without that vote, there would likely have been no Constitution agreed to and signed on September 17, 1787.