I have day time foals all the time!!! I even foaled out 2 outside mares this year and both of them foaled in the day time! I get at least a few day time foals each year. One year I was expecting 5 foals and 4 of them were day time babies!
So my answer is, all the time!!!
I'm thinking that it depends (maybe) on their comfort and feeling of safety of their surroundings. No need to be secret at night? My mares all depend on "mom" being there.
More than half of mine have foaled during the day, last year all 3 foaled outside during the day. Britt likes to have hers just after dinner (around 8.00pm)
My girls like to foal outside so they have to do it during the day before I put them in their stables
I've had 8 foals and 2 were during daylight hours, one at 930am and one at 530pm. I have been there for all 8, so I'm fine with them foaling during the day, makes for an easy sleep that night!
Out of 26 foalings, I've had 8 born during the day (between 5am-5pm), 12 between 7pm-11pm. 6 between 11:50-3am. My girls know for sure I will be around in the evening, and that is typically when things get quiet here on our farm.
Very few! My mares have just never liked to foal during the day. Have only had a couple and it's usually the really sneaky mares who have the foal right after I let them out of their stalls for the day and am busy doing chores so am not watching them.