If you were happy for the message to be displayed within your instance itself you could use the 'Agile Sprint Health Gadget' on your dashboard.
Edit your 'System Dashbaoard' from cog-icon -> 'System' and add 'Agile Sprint Health Gadget'
It highlights the days remaining along with scope change and story estimates.
While you cannot query the sprint end date within JQL, you could use the following:
sprint in (openSprints())
Save the above as a filter (including an other restrictions your team requires). Once saved you can access the 'Details' hyperlink on the filter and click 'New Subscription'. You can set the filter subscription to be delivered to any groups on your instance on a schedule you prefer.
The result is users will be aware of each open sprint as regularly as you require.
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