I think mid morning may well be best
I think the cathedral is most atmospheric when the light is beginning to fade, but it is beautiful in bright sunshine too. We visited in June and the cathedral was busy pretty much all the time.
We went around 10am and in August so it was height of season. I don't recall the time of day being a problem.
We went early in the day which was fine. Around 10. Wrap up warm!
The best time to visit is during the week, you have to check the times there first because there was some reconstruction going on
The best time is before 11 am or after 2 pm during the week.
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