Are there roses in bloom in may?

There were a few..but in July and August they are in full bloom

There were a lot in bloom in August, so I'd think May might be a little early for them.

I saw a couple last time I been there.

I only saw maybe 5 rose blooms when I was there a few days ago.

A few right now. There should be more later.

Is it worth viewing pay visit during Thanksgiving?

Hardly any, but still worthwhile for the rhododendron and other plants, just not roses. Best garden in April/May in PDX IMO is Crystal Springs Rhododendron Garden near Reed on Eastside.

We were there June 3rd and it looked like it was almost full bloom