When a browser connects to a secure site it will retrieve the site's SSL Certificate and check that it has not expired, has been issued by a Certification Authority the browser trusts, and is being used by the website for which it has been issued.
If it fails on any one of these checks the browser will display a warning to the end user. Make sure you renew your certificate with time to spare.
Some customers may look at the date and be concerned if it expires too soon.
Check for the vendors who, when you purchase before the expiry date, will actually add remaining days to your certificate for free to avoid this problem.
The other alternative is to make sure vendors purchase a multi-year certificate to minimise set-up costs and demonstrate to their customers that the business will be around in the years to come.