When to apply pre-emergent in MN ?

I'd appreciate some advice on best time to spread pre-emergent in MN (Twin Cities) and on what to apply. Looking for general crabgrass/dandelion prevention. Cut grass and power raked for the first time last Friday.
Thanks,
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Google gave me this
http://www.linders.com/tips/care-article.html?id 6&table=main
Folklore wrote:

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Get it down before the forsythia stops blooming...Scotts Halts or Dimension are good grass weed preventer's.
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