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Definitely go to the reception.
Try to reschedule the other callback.
I would not reschedule the other callback (especially if you would be willing to work at that firm if your choice firm rejected you). A reception probably isn’t going to make or break you. I would politely explain why you cannot and apologize. At the end of the day, you probably want a job, and rescheduling a callback is a poor look.
In what world can’t you do both reception and callback? Most receptions start at least at 5:30 and most callbacks end by late afternoon… i feel like it would be a special scheduling circumstance that would prevent you from actually being able to do both.
Firms that do receptions to so to test the sociability of a candidate and is part of the vetting process. I received an offer from a firm and was told by associates there that a large reason I received my offer was because of me performance at the reception (obviously I had the grades and all that too, but that’s assume at offer stage). Receptions or a key part of the process and for many more firms these days are essential to vetting as they step away from OCI.
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