Anomalous traffic

One of my GA4 accounts trundles along with around 250 visits a day. Once a month, for the last 6 months one day around the middle of the month (not the same day) we get a spike of 5-10,000 visits.

GA4 says "traffic from known bots and spiders is automatically excluded" and "At this time, you cannot disable known bot traffic exclusion". GA4 also notes that there is an "Anomaly Detected" (me too).

These visits are all marked as 'direct' and all coming from Ireland

How can I find out more about this traffic?