If my certificates are expiring tomorrow, Can I sail today ?
I would assume that you are talking about "certificate of competancy" and not the ship's certificates.
if you are about to sail for a voyage that would last more than one day, the answer to your question would be "NO'
If the voyage is only for one day, no doubt you can sail.
There is this thing that says, if the certificates are expired during the voyage, the certificates can considered to be valid until the completion of voyage. But this is only for unavoidable circumstances. Circumstances like unplanned anchorages.
But if you are sailing for a voyage with complete knowledge that your certificates would expire during the voyage, it would not be allowed unless a dispensation/extension is received from the issuing country.
After the extension is received, the company then also need to apply to the flag of the vessel for extension of flag dpcuments before you can sail for the voyage.