What is basis for when electoral votes are cast?

Answer 1

The entire electoral college does not meet together in one place. Electors meet in their respective state capitals (electors for the District of Columbia meet within the District) on the Monday after the second Wednesday in December, at which time they cast their electoral votes on separate ballots for president and vice-president. Each state then forwards the election results to the President of the U.S. Senate, the Archivist of the United States, the state's Secretary of State, and the chief judge of the United States district court where those electors met. A joint session of Congress takes place on January 6 in the calendar year immediately following the meetings of the presidential electors. The electoral votes are officially tabulated at the joint session of Congress and the winner of the election is officially declared.

Answer 2

Since the election of 1824, most states have appointed their electors on a winner-take-all basis, based on the statewide popular vote on Election Day. Maine and Nebraska are the only two current exceptions, as both states use the congressional district method. Maine and Nebraska use the "congressional district method", selecting one elector within each congressional district by popular vote and awarding two electors by a statewide popular vote.

Answer 3

There actually isn't a constitutional basis. Article 1 Section 2 references electors but makes no provision for the electoral college.