When you are awarded Social Security disability benefits, you do not receive benefits beginning on your onset date (which is the date you became disabled, and were not working). Rather, there is a 5-month “waiting period” before benefits accrue.
As a practical matter, however, it is often a 6-month waiting period. This is because the waiting period is 5 full months of disability. So if you have partial month of disability, that month simply doesn’t count. I have had clients with an onset date on the 2nd day of the month, and that whole first month of disability did not count. So the effective waiting period can be as long as six months before benefits begin.
The best possible onset date in this respect is the first or last day of the month.