There is a hadeeth in Sunan Abi Dawood with an acceptable isnaad from ‘Aaishah (may Allaah be pleased with her) which states that the Prophet (peace and blessings of Allaah be upon him) set a day when the people should go out. ‘Aa’ishah said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) went out when the rim of the sun appeared…” The hadeeth clearly indicates that a day was set when the people were to go out to pray Salaat al-Istisqa’, even though it does not name this day. More than one of the scholars have stated that it is mustahabb to choose Mondays and Thursdays, because deeds are shown to Allaah on those days, and because these are good days for fasting. So if the Muslims combine fasting and praying for rain, their du’aa’s in this case are more likely to be accepted.
It may be said that it is not prescribed to single out these two days rather than others, because that was not proven from the Prophet (peace and blessings of Allaah be upon him) or from any of the Sahaabah, and this is the correct view. So it is not prescribed to choose one day rather than another without there being any text to prove that. What is prescribed is to choose a day on which the people should go out, which may happen to be a Monday or any other day, according to what is in the people’s best interests and according to what is convenient for them.