What is the least crowded time of the day to visit Giverny ?

I visited mid-morning in May a few years ago and it was already pretty busy. I should have gotten the earliest train from Paris to Vernon, but it was raining in Paris when I woke up so I wasn't even going to go until the rain stopped. Doing it again, I'd get to Giverny first thing when it opens and buy a ticket online ahead of time so I could skip the line...

I would choose the morning.. it only gets hotter and more crowded as day goes on... not less.

The least crowded time of day will most likely be right after they open in the morning. Try to be there at least 1/2 hr before opening time. With that being said, in June it will be light until quite late so late in the afternoon may also be a good time. They are open until 6 pm so around 4 pm might be a good time. If you're interested in good photos of the flowers and ponds then as early or late as possible are the best times.

Yes, school is in session in June.

Thank you very much to all. Closer to travel I'll check the weather ( could be rainy day(s) )

Be advised that the river cruises stop there, disembarking a couple of hundred people, plus bus tours. When we were there in mid-September it was very crowded with several large groups, so do arrive early.

As a general rule on popular sights, I try to arrive 30 minutes before opening. Getting in right when they open will give you the most time with the fewest crowds.

Do purchase your ticket in advance. You will be able to enter at the bottom of the garden, group entrance. Go right under the road to the water garden first, then back up to the house, garden and gift shop. Don't miss the Orangerie Museum while in Paris to see the huge paintings of this water garden.