There will be three prizes so feel free to pick a date that two others have - or pick a time different from the others on that date. Almost all of my posting is done between about 7am and midnight UK time - currently UTC+1 which it will be until well after I'm likely to have made 6000 posts. We go back to UTC/GMT at 3am on the last Sunday in October.
Saturday, April 21, sometime between 10:00 and Midnight US CDT.
<edit>I'll revise my original prediction to the morning of Sunday, April 22, to reflect the fact that Senor Don will still be asleep at that time. I'll guess ~9:00 am, BST.
Four score and still doing well, I hope. Foy, on the other hand . . . well, I'm just trying to make it to three score minus seven . . . or, two score plus thirteen or . . . whatever the damn score is.