Cost Anomaly Detection

Hello.

Is the reminder time for the daily summary fixed at a certain time or will it be reminded once it exceeds?

As mentioned in the documentation below, Cost Anomaly Detection is run approximately 3 times a day.
Additionally, due to the use of Cost Explorer data, there may be a delay of up to 24 hours.
Therefore, it may take some time before it can be detected.
https://docs.aws.amazon.com/cost-management/latest/userguide/manage-ad.html

After your billing data is processed, AWS Cost Anomaly Detection runs approximately three times a day in order to monitor for anomalies in your net unblended cost data (that is, net costs after all applicable discounts are calculated). You might experience a slight delay in receiving alerts. Cost Anomaly Detection uses data from Cost Explorer, which has a delay of up to 24 hours. As a result, it can take up to 24 hours to detect an anomaly after a usage occurs. If you create a new monitor, it can take 24 hours to begin detecting new anomalies. For a new service subscription, 10 days of historical service usage data is needed before anomalies can be detected for that service.

Is the threshold set for every day or a single month?

Since you can set a daily summary, I think it is possible to detect it on a daily basis.
https://docs.aws.amazon.com/cost-management/latest/userguide/getting-started-ad.html

I still have the remaining amount in my points deduction coupon. Is this the reason why the abnormal reminder is not detected?

In my experience, I have set up Cost Anomaly Detection in an AWS account that has credits applied in the past, and I have received notifications when an anomaly occurs.

My requirement is to remind me when I spend more than 100$ per day. How should I configure it?

Cost Anomaly Detection is a mechanism that detects when costs suddenly increase or there are some abnormalities in costs.
For example, if you set the threshold at 100 USD for an AWS account that consistently charges around 100 USD daily for usage, you would not be notified if the difference is less than 100 USD among the activities detected by the cost anomaly detection.
In this case, you will be notified when 200 USD is used.
https://docs.aws.amazon.com/cost-management/latest/userguide/getting-started-ad.html

So, in your case, I think it would be better to set AWS Budgets instead of Cost Anomaly Detection.
https://docs.aws.amazon.com/cost-management/latest/userguide/budgets-create.html