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Alexa can play the song using an Alexa routine ( not a SmartThings routine)
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you can trigger an echo routine using a virtual contact sensor. See the FAQ for how to do that
FAQ: Can I trigger an Echo Action without Speaking to It?
- now all you need is something to turn on the virtual sensor on the date that you want. Lots of choices for that. Webcore is a good option, but you can also just use Google calendar with IFTTT.