Is it permissible for Sha‘ban to be 31 days, and is fasting tomorrow permissible, knowing that it is the 30th of Sha‘ban and tomorrow is the completion of the month due to the crescent moon not being sighted?

The lunar month is twenty-nine or thirty days and does not exceed that. It is obligatory to sight the crescent moon on the night of the thirtieth of Sha'ban. If it is sighted, people will fast the next day. Otherwise, they complete Sha'ban as thirty days and then fast. Sha'ban can never be thirty-one days. If an error occurs in the calendar, leading to such a belief, then it is obligatory to conform with the people in their fasting and breaking of the fast. However, if they actually count Sha'ban as thirty-one days, it is not permissible to follow them, and it is obligatory to fast the day mistakenly claimed to be the thirty-first of Sha'ban, because it is actually the first of Ramadan.