Hi @BdeBruyn,
You can find this right here when creating a workflow 
Details are explained here: Create 'Specific date' or 'Contact date property' contact-based workflows
You can also use a regular contact-based workflow and use the “Delay until calendar date” option.
By checking the box at the bottom, you’ll get a branch. The left branch for people who arrive at the delay before the date, the right branch for contacts who arrive there after the delay. This looks a little bit more intimidating but can help reduce the number of event workflows, by integrating reminders directly in the workflow that also sends the registration confirmation. Let me know if you want more detail here.
Best regards!