It has nothing to do with capture day. yes there is a random factor. However, each time a stallion produces a WWW or anything above EEE the chances of him producing another slowly go down. So by the end of the month if he's bred 17 times and produced multiple WWW it's unlikely you'll get one
I do 2yr olds so I can start training and like Vetta said, it's so late already in the yr it's unlikely if anything from a decent capture produces anything more than a PPP