Hey kgs, I had asked this very question to our team in April and heard that no date had been set yet. However, since it's been a month or so, I've reached out to our team to see if there have been any updates. Stay tuned!
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I am staying at the Delta Prince Edward in Charlottetown in June and also had the same question. I searched Marriott.com and all it says is watch for details.
Haven't heard about Delta coming into Marriott fold, but if need information about this part of Eastern Canada, might check this post: Prince Edward Island & Nova Scotia - Canada
It appears that Earning and Redeeming Points and Nights at Delta hotels will not begin until early 2016 but Marriott Rewards Elite Members will receive some benefits at Delta Hotels beginning June 18th.
This online article has more information: http://thepointsguy.com/2015/06/marriott-rewards-at-delta-hotels/