Submit well ahead of time. It's extremely important to submit your grant proposal before the deadline. You know how it goes - if anything can go wrong, it will and there is a lot that can go wrong with the submission process. One of our proposal writers once suffered a power outage during a proposal submission when a squirrel chewed into the transformer of the power pole outside his office - talk about bad luck!
I have never worked with a grant-making agency that accepts late submissions, and it would be a shame to invest your time and energy writing a grant, only to lose out due to forces beyond your control.
If submitting by mail...
Most federal agencies have switched to a paperless grant system and will no longer accept paper copies without prior approval. (Ironically, the Environmental Protection Agency still accepts only hard-copy proposals). If you're working with a grant-making agency that does accept proposal submissions through mail, double-check the postage and get it in the mailbox early. Some agencies will accept grants that are postmarked by the deadline date, others require it to be in their hands by the deadline, no exceptions. Read through the grant guidelines to see the agency's policy, and if you can't find the answer, ask them. Also, double check the address. Some agencies have different addresses for U.S. mail than for other couriers.
If submitting by bit (electronic submission)...
Electronic grant submission is more the norm now, and it's worth your time to explore the grant-making agency's grant submission website several weeks before the deadline date. The day the grant is due is not the day to figure out how the website works, no matter your technological hubris. Some proposals require registration at multiple sites, each of which can take a week or so to make happen.
Submit well ahead of time (it's worth saying again). As a company policy, we upload our grant proposals onto the submission website at least a few days before the deadline. We then check every uploaded file to ensure it's the right file, in the right place. We also have at least two pairs of eyes check the vital details of the grant's cover page before submitting - is the DUNS number correct? Is the EIN number correct? The contact's email address? This way, if we find mistakes, we have time to fix them.
Our power-outaged grant writer was able to submit successfully when the power came back up since it was before the deadline, and he lived on to submit many more. Not so lucky was the squirrel - that transformer was the last dispenser of publicly available resources it ever dealt with.
Interested in more grant writing advice? Take advantage of The Grant Helper's free grant writing consultation.