应该说on the first day还是at the first day答:at a time(在某时), at a wedding(婚礼), at all times(一直), at any moment, at any time, at Christmas, at dark (天黑时), at dawn(在黎明), at daybreak, at dinner-time, at dusk(在黄昏), at Easter, at first sight,(一见到), at first, at last, at midnight, at ...