With the reminder function in Slack you can always have a heads up on 4:20. To set a reminder you use the /remind keyword. The exact syntax looks like:
/remind [who] [what] [when]
So to be reminded about 4:20 you would write:
/remind me "Yo, heads up. 4:20!" at 4:20pm
To take it a little further, you can have Slack remind a whole channel with a graphical readout everyday:
/remind #theHeads :four::two::zero: everyday at 4:20pm
To get your apple computer to verbally remind you everyday at 4:20 that is 4:20, you need to set a crontab with a say command.
In your terminal run the command: sudo crontab -e. Then enter the following line:
20 16 * * * say -v fred 'what time is it?'; say '4 20'