ORACLE to_char (sysdate, ' D ')

DDD is the day within a year of the number of days, D is within a week of the first day, DD is within one months of

Dy:day of week abbreviated Mon, Tue, Fri
Day:day of week spelled out Monday, Tuesday, Friday
D:day of Week (1–7) 1,2,3,4,5,6,7--Note: The 1th day of every week is "Sunday"
Dd:day of Month (1–31) 1,2,3,4 ... 31
Ddd:day of year (1–366) 1,2,3,4 ... 366

From

https://www.cnblogs.com/tider/archive/2009/05/07/1451296.html#undefined

ORACLE to_char (sysdate, ' D ')