But there was no way to hold Sumter beyond April 15 without resupplying it, and so that's what Lincoln attempted, and notified South Carolina Governor Pickens on, April 6.
I hope you aren't still using that discredited almanac you've used before for "facts" about the war. April 6? Try this online source: April 8
Oh, and why on March 5, the day after his inauguration, did Lincoln secretly start trying to reinforce Fort Pickens and break the truce without informing the other side, possibly beginning the war there like Adams and Meigs said? Was Fort Pickens running out of food like Sumter?
All my sources, including this one, say that Lincoln sent the message to South Carolina Governor Pickens on April 6, some say delivered April 8 by US State Department clerk, Robert S. Chew:
They also say that in response, by April 8 Confederates were making final preparations to assault Fort Sumter, and on April 9, Davis' cabinet endorsed his orders to capture Sumter before the resupply could arrive.
Your source for this data is what?