How to schedule a message in Slack

Type your message in the message field

Click the down arrow "Schedule for later" next to the Send icon at the bottom right

Select one of the offered options or click "Custom time" to set a different one

Click on the down arrow in the first box to select a different day

Select a day in the calendar

Click on the down arrow in the second box for time

Type or select a time from the list

Click "Schedule message"