I quoted an older post of mine above with a link to a source that seems to back up the "no assignment during trading hours". I'm definitely not saying that it hasn't happened to people here, but would be interested to know the actual mechanics as it has some very relevant implications. Maybe when your broker assigns you midday, it's actually from "the night before"? Something along the lines of the following scenario:
- I sell a call on Monday.
- Tuesday, someone, somewhere exercises the same call
- Tuesday evening, the OCC clears the exercised call and informs brokers that they need to assign
- Wednesday mid-day, the broker gets around to assigning one of their clients (me).
- Wednesday mid-day, I get an assignment notification and my shares disappear.
Does that seem reasonable? For all intents and purposes, it means that you can be assigned by your broker at anytime, even though the clearing process doesn't happen at the same time. This is all guesswork on my part, so YMMV.