Companion Certificate expiration

Got an email from Delta. I had booked a flight with my companion cert but had to cancel. They were advising me that it is again available for use. I am a little unclear on the expiration:

|| || |Your certificate xxxxxxx has been added to your SkyMiles account. Visit delta.com or the Fly Delta App to redeem before January 31, 2027, 12:00AM EST.|

Does that mean I have to book something before this date, or do I have to be on the plane before this date?