The cowboy's horse is named Sunday. By the way, the cowboy's brother's horse is named Saturday. The cowboy's brother arrived on Saturday on Saturday, stayed for 8 days, and left on Saturday on Sunday, or did he leave on Sunday on Saturday ?
I think the mode of transportation takes precedence over temporal specification in the dictation of one's departure. Therefore, to answer Arthur's point, I say the former of the two is correct wrt one species of conventional dictation.