It's all about the dividend payment, i.e. ex-dividend date. Always check ITM calls when the stock is about to go ex-div. And, yes, assigments happen overnight hence you don't find out about until the next day. This is where the whole early call exercise prior to ex-div and shorts being responsible for the dividend comes from.
Funny thing my calls were NOT ITM at the time of assignment thats why I was so surprised but obviously close enough to the money that warrented assignment by someone.
The maths behind early call exercise are pretty simple. If the dividend is more than the cost of carry to expiry plus the price of corresponding put then the call will be exercised.
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