A health certificate is a dated document with a short working life, so the flight problem is really a diary problem. The certificate must be recent enough when you travel, which means the vet visit cannot happen whenever it happens to be convenient. Work out the window first, then place the appointment inside it with room to spare, because a certificate issued too early is as useless at check-in as one you forgot entirely.
The three timing rules to check
First, the airline's rule: carriers set their own requirement for how many days before departure the certificate may be issued, and it can differ between cabin and hold travel. Second, the destination's rule: arriving somewhere new can add requirements beyond the airline's, including particular vaccinations being current or additional paperwork for entry. Third, the return rule: a trip longer than the certificate's validity can leave you needing a second examination and certificate before you fly home. Read all three before booking, because each one changes where the vet appointment belongs.
Counting backwards into the window
Once you know the window, count backwards from departure day and choose an appointment date comfortably inside it rather than at either edge. The last possible day leaves no room if the vet is sick, a form needs correcting or your dog is off colour and the examination should wait a day. The earliest possible day risks the certificate ageing out if your flight is rescheduled. A date in the middle of the window absorbs a delay in either direction, which is exactly what you want for a document you cannot easily replace at the airport.
What the vet needs from you at that visit
Bring the airline's stated requirements in writing, your destination details and your exact travel dates, so the vet completes the certificate against the real rules rather than a standard template. Confirm your dog's microchip number and vaccination dates match what is already on her records, because a mismatch between the certificate and the underlying file is a common reason for a document to be questioned. Ask for a copy for yourself at the appointment rather than chasing one later, and check names, dates and chip numbers before you leave the building, while a correction is still a small job.
Diarising the whole sequence
Put three entries in your diary, not one: the date the window opens, the appointment itself, and a check a day or two before travel confirming the certificate is packed and still valid for both legs. If you are travelling again within the year, note which parts of the process took longest, since the next booking will run faster with that knowledge attached to the record. Travellers who treat the certificate as a sequence rarely meet surprises at check-in, because every deadline was met while it was still easy to meet. If your dog will travel in the hold rather than the cabin, confirm whether the same certificate and timing cover that arrangement, because some carriers handle the paperwork and check-in process differently depending on where the dog travels.
Handled this way the paperwork stops being the stressful part of the trip, and KinsLe's Pet Passport keeps the certificate and vaccination records together in one portable packet for check-in staff and vets at either end.