SMS Blackout days are those days when you cannot send text messages/SMSs for free or as per your promotional tariffs and you will have to incur the normal cost for sending SMS on such black out days.
SMS Blackout days is actually made effective in the Indian telecom industry to avoid unwanted congestion/failure of networks due to heavy network usage during festivals and occasions when users usually send lot of greetings and wishes via text messages.
Blackout days are usually selected by individual operators and do vary from operator to operator and region to region. So you need to check out with you operator whether a particular day has been blacklisted as SMS blackout day or not.
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