The 30-Day Month
If no witnesses came on the thirtieth day—either because the moon had not been "reborn" yet, or because it was not visible—then the next day, the thirty-first day, was automatically declared Rosh Chodesh, retroactively rendering the previous month a malei month.
Members of the Sanhedrin would go to a highly visible location, where they would partake in a celebratory meal to signify the new month. No fires were lit that night. The new month is always either on the 30th or 31st day; if they hadn’t lit fires the night before, it was understood that the new month started on the 31st day.