Originally Posted by Zenithale

... Just a noob question, can participate to a Dream Fest during the 30 min or you absolutely need to be present at the first second of the Entry Time? I ask that because after a lot of changes of Entry Times I was able to get a Dream fest at 00:00, then when that was the finally time (after few hours of waiting), I came few seconds after the beginning and the Participate button stayed grey...

You can participate at any time during the half hour of the event, even during the last 10 seconds (though I highly advice against making it so close because of the popups and long loading times).
I've joined Dream Fest several times during its last couple of minutes, and usually lost horribly on those occasions because the opponent had already worn out my team. But also won a couple of times because of that, because the opponent figured he/she already won because I was offline, and left. Only for me to completely ruin his/her team during the last few minutes.