An example if you want you schedule to pop up for every hour.

1. Install Osmo and add the task you want to its timeline.
sudo apt-get install osmo
2. Check you display number using command
xdpyinfo | grep display
… if comand prints ”name of display: :0.0” , add the next command to the crontab
crontab -e ## choose nano when asking
… add the line
0 * * * * export DISPLAY=:0 && /usr/bin/osmo