For most of the history of Social Security, all monthly payments of benefits came out on the third of each month. That changed in 1997 so that the Title II payments come out on three dates each month. The date you receive your payment depends upon the day of the month upon which you were born.
The Social Security Administration's statement at the time this change was made gives an insight into the level of service that was then prevalent at the agency. This is from a notice published by Social Security in the Federal Register on February 11, 1997 (emphasis added):
SSA’s current practice of paying 47 million beneficiaries within the first 3 days of each month results in a large surge of work during the first week of each month. This surge includes a large number of visitors to field offices and calls to our toll-free 800 number to report nonreceipt of a check, question the amount paid, or ask about other payment-related issues. Approximately 9 percent of all calls during check week concern nonreceipt, compared to 3 percent during the rest of the month. As an example of the surge that occurs around the current payment days, on April 3, 1995, 1,091,282 calls were placed to SSA’s 800 number. On April 14, 1995, the number of calls placed to our 800 number decreased to 229,022. It is important to beneficiaries and customers to be able to reach SSA with fewer busy signals, and we have pledged to enable callers to get through to the 800 number within 5 minutes of their original attempt. However, in fiscal year (FY) 1994, during peak periods, customers encountered busy signals on SSA’s 800 number 40–63 percent of the time and had to wait more than 5 minutes to get through about 30 percent of the time. ...
Our goal is for our customers to have minimal waits for service when visiting a Social Security field office. Today, SSA does not always meet this goal. In FY 1994 there were 24 million visitors to our field offices. While the average wait during check week for individuals with an appointment was 8 minutes, some individuals with appointments had to wait over 2 hours. Thirty-two percent of the visitors to our offices without appointments in FY 1994 (typically people who have questions related to their payments or who want to report payment delivery problems) had to wait more than 30 minutes after arriving to be served. The average wait during check week for individuals without appointments was 16 minutes, although some individuals without appointments had to wait over 3 hours. ...
The agency already felt it was under customer service stress way back in 1997 but the level of stress was vastly less than it is now. Adequate service has been defined downward dramatically since then.