, I see from another post of yours that you will be flying from the United States to Spain. This is important to know because the kind of QR code you get from the Spanish health department's website depends on a number of factors. Under their current system, people get either a Fast Control QR
code or a Documental Control QR
The earliest that people can receive the Spanish QR code is 48 hours before travelling. This is when they can complete
the application form. Once you have completed the form correctly you will receive the QR code immediately.
At the time of writing (27 August 2021), travellers flying directly from the USA to Spain do not have to provide any proof - no proof of vaccination, no tests. They receive a Fast Control QR code.
This can change, and this is the reason why you can already now start filling in the application form for your October flight but you cannot complete it weeks or months before the start of your trip. Every traveller needs to stay up to date with the entry conditions that apply to their trip.