As obvious as can be. The earliest similes for patently obvious were plain as a packstaff or pikestaff (that is, the staff on which a peddler or hobo carried his bundles), used from the fifteenth century, and plain as the nose on your face, from the sixteenth century. The former is obsolescent, at least in America; the latter is still very current, heard even more than plain as day, a nineteenth-century locution.