I got married on Labor Day weekend and then had our honeymoon at DLR , this was last year. We left for SoCal on labor day monday, and had our first day of DL that night and then the whole week up until that coming Sunday. The weekdays were not bad but it slowly started to get a tad crowded as Thursday rollled by.
Like the poster said before, getting into the parks early will ensure that you at least get all your fave rides in before the crowds come in. The park was open by 8am and closed at midnight every night we were there. We were able to do our faves on Saturday and Friday because we got there at gate opening.
Weather wise, it was hot and a little bit humid (indian summer). Your alternate date of June sounds like a good plan as well because the weather isn't so bad then and not that many schools out yet (though they will be that week). Those schools that get out the first or second week of June, usually have kids starting DL vacations later in the summer. This is hit or miss but June is a great month I think. By Labor day weekend, everyone is trying to cram in their last minute vacations.
Good Luck!