The World Federation One Stop Fiqh

Ruling 1593

If a junub in the month of Ramadan forgets to perform ghusl and remembers after one day, he must keep a qaḍāʾ fast for that day. If he remembers after a few days, he must keep a qaḍāʾ fast for all the days he is certain to have been junub on. For example, if he does not know whether he was junub for three or four days, he must keep qaḍāʾ fasts for three days.