intercident

Coming or falling between; happening accidentally.

But it is found by ſad experience, that ſhe rouſes herſelf up to make a criſis, not only upon improper, and, as phyſicians call them, intercident days, […] vvhich ſeldom afford any criſis, and much ſeldomer a good one; but alſo vvhen there appear not any ſigns of coction, or at leaſt of due coction, and by theſe unſeaſonable attempts vveaken the patient, and increaſe the malady, or perhaps make it ſpeedily mortal.

A day that falls between other days of importance.