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Isolation upon infection with Covid should be 10 full days, starting with the first day of symptoms, which you have indicated was on 12/8/21. Therefore, his period of isolation is to be until 12/18/21 (this weekend). By the time of 12/24/21, as long as he is feeling well (has not caught influenza or some other “bug”), then he should be okay to join in the family festivities. Here is the CDC guidance on isolation and quarantine. I also add that Covid cases have sharply risen in the past few days and therefore I would recommend that all family members try to think about getting tested prior to these large planned gatherings. At least 40% of Covid infections are “silent” but can still transmit to others and if it is an older adult, someone with chronic medical problems, or an immunocompromised person then it can be a very series infection or deadly.