Is it permissible to designate specific days for going out in the path of Allah to the exclusion of others, knowing that whoever does not go will miss the opportunity?

The Tablighi Jamaat, like other groups, aims to invite people to goodness. It employs specific methods of da'wah (invitation) and means of encouragement. Despite their good intentions and great effort, their ijtihads (independentreasoning) are not infallible and may at times deviate from what is correct. Specifying certain days for going out (for da'wah) as an obligation has no evidence. However, specifying a number of days for the purpose of organization and facilitation is permissible, which is what the group does.