You do not need to get your second dose exactly 21 (for Pfizer) or 28 (for Moderna) days after your first shot. The Centers for Disease Control and Prevention (CDC) have advised that the second should be given no more than 8 weeks after the first dose. However, the CDC has also indicated that you do not need to start the shots over again if you go beyond the 8 week window for the second shot. It will still be effective.
To learn more about the timeline of COVID-19 vaccinations, please visit Frequently Asked Questions about COVID-19 Vaccination.