Best time of day to take a vitamin

Exactly what the title says.

My son (ADHD/depression thanks to the lack of sun here, etc) refuses to take medications and the next option is to try a multivitamin that his doctor suggested.

I’ve been one who takes mine whenever I can remember and have never noticed a difference.

So, is there an actual best time to take them? I found mixed info on Google and forgot to ask the doctor.