Experienced exactly the same problem a few months ago: you need to understand "May 31" as "friday-evening-wich-is-before-May-31, business hours" so in this case May 30 4 pm...

So I called them the day after and they allowed me to book at the promo rate without problem.

For the little story, when I saw my booking was not possible, it was late in the evening, so no possibility to call them, I send them a message with the FB contact form... and I received a phone call (!)... three weeks later
It was nice from them, but a bit late... It was 15 days I returned from my trip!(promo-award ancient version)