Go to -> Appearance -> Widgets.
Drag the Events widget from available widgets to the right widget bar.
See screenshot of that available widget :
https://help.squat.net//today-event-widget-sidebar/Events-widget-available.png

Then configure it by showing 1 event and editing the event link code.
See screenshot of that configuration :
https://help.squat.net//today-event-widget-sidebar/Show-events-for-today.png

A screenshot of real life usage here below :