I've hunted NW Nebraska during the Memorial Day weekend several times. I've hunted end of April, middle of May, and end of May. I've heard birds every time. The April hunt turned into a blizzard and freezing temps, so I'm not sure what decent weather would have produced. Early to Mid-May probably had more gobbling toms than the end of May but we struggled with henned up birds. Late May had less gobbling, but if we covered plenty of ground we found birds that wanted to play.
I believe that really hot weather will start to shut them down. We experienced that on one hunt the first couple days and found a group of toms that had no interest. We lucked into a right place right time situation and killed two of them. The weather cooled off the next few days and we found several gobbling birds and killed a couple more.
If I find a way to make it over there over Memorial Day this year, I'll do it without hesitation.
Last year I hunted about the second week of May and it was raining most of the hunt and I blew one opportunity to kill a tom.