How long Dynatrace is working on this environment? In general AI should learn amount of traffic and do not alert you about low traffic. If Dynatrace is working for less than couple of weekend I suggest to give him some time. In general after week should detect such traffic drops automatically. From my experience such alerting is gone some times after few weeks.
As I understand your alerts came from application. You can make globally anomaly detection settings or overwrite it in single application configuration. As you can here traffic drops are set on 50%. Unfortunately there is no option for auto switching configuration just for weekends.