Add ESQL DAY_NAME function

docs/reference/query-languages/esql/_snippets/lists/date-time-functions.md

@@ -3,4 +3,5 @@
 * [`DATE_FORMAT`](../../functions-operators/date-time-functions.md#esql-date_format)
 * [`DATE_PARSE`](../../functions-operators/date-time-functions.md#esql-date_parse)
 * [`DATE_TRUNC`](../../functions-operators/date-time-functions.md#esql-date_trunc)
+* [`DAY_NAME`](../../functions-operators/date-time-functions.md#esql-day_name)
 * [`NOW`](../../functions-operators/date-time-functions.md#esql-now)

docs/reference/query-languages/esql/functions-operators/date-time-functions.md

@@ -28,6 +28,9 @@ mapped_pages:
 :::{include} ../_snippets/functions/layout/date_trunc.md
 :::
 
+:::{include} ../_snippets/functions/layout/day_name.md
+:::
+
 :::{include} ../_snippets/functions/layout/now.md
 :::
 

x-pack/plugin/esql/qa/testFixtures/src/main/resources/date.csv-spec

@@ -1836,3 +1836,97 @@ y:date                   | c:long
 1987-07-01T00:00:00.000Z | 1
 1987-08-01T00:00:00.000Z | 2
 ;
+
+dayNameRowTest
+required_capability:fn_day_name
+// tag::docsDayName[]
+ROW dt = to_datetime("1953-09-02T00:00:00.000Z")
+| EVAL weekday = DAY_NAME(dt);
+// end::docsDayName[]
+
+dt:date   | weekday:keyword
+1953-09-02T00:00:00.000Z | Wednesday
+;
+
+dayNameSimple
+required_capability:fn_day_name
+from employees 
+| sort emp_no 
+| keep emp_no, hire_date
+| eval day = day_name(hire_date) 
+| limit 1;
+
+emp_no:integer | hire_date:date             | day:keyword
+10001          | 1986-06-26T00:00:00.000Z   | Thursday     
+;
+
+dayNameTruncate
+required_capability:fn_day_name
+FROM employees
+| EVAL a = hire_date
+| RENAME hire_date as b
+| WHERE a > "1987-01-01" and a < "1988-01-01"
+| EVAL y = date_trunc(1 month, b) 
+| STATS c = count(emp_no) by y
+| EVAL week_day = day_name(y)
+| SORT y
+| KEEP y, c, week_day
+| LIMIT 5;
+
+y:date                   | c:long   | week_day:keyword
+1987-03-01T00:00:00.000Z | 5        | Sunday
+1987-04-01T00:00:00.000Z | 3        | Wednesday
+1987-05-01T00:00:00.000Z | 1        | Friday
+1987-07-01T00:00:00.000Z | 1        | Wednesday
+1987-08-01T00:00:00.000Z | 2        | Saturday
+;
+
+dayNameNestedCall
+required_capability:fn_day_name
+from employees 
+| sort emp_no 
+| eval first_day_of_month = day_name(date_trunc(1 month, hire_date))
+| keep emp_no, hire_date, first_day_of_month
+| limit 10;
+
+emp_no:integer | hire_date:datetime       | first_day_of_month:keyword
+10001          | 1986-06-26T00:00:00.000Z | Sunday
+10002          | 1985-11-21T00:00:00.000Z | Friday
+10003          | 1986-08-28T00:00:00.000Z | Friday
+10004          | 1986-12-01T00:00:00.000Z | Monday
+10005          | 1989-09-12T00:00:00.000Z | Friday
+10006          | 1989-06-02T00:00:00.000Z | Thursday
+10007          | 1989-02-10T00:00:00.000Z | Wednesday
+10008          | 1994-09-15T00:00:00.000Z | Thursday
+10009          | 1985-02-18T00:00:00.000Z | Friday
+10010          | 1989-08-24T00:00:00.000Z | Tuesday
+;
+
+dayNameNestedCall
+required_capability:fn_day_name
+from employees 
+| sort emp_no desc
+| eval first_day_of_month = to_upper(day_name(date_trunc(1 month, hire_date)))
+| keep emp_no, hire_date, first_day_of_month
+| limit 10;
+
+emp_no:integer | hire_date:datetime       | first_day_of_month:keyword
+10100          | 1987-09-21T00:00:00.000Z | TUESDAY
+10099          | 1988-10-18T00:00:00.000Z | SATURDAY
+10098          | 1985-05-13T00:00:00.000Z | WEDNESDAY
+10097          | 1990-09-15T00:00:00.000Z | SATURDAY
+10096          | 1990-01-14T00:00:00.000Z | MONDAY
+10095          | 1986-07-15T00:00:00.000Z | TUESDAY
+10094          | 1987-04-18T00:00:00.000Z | WEDNESDAY
+10093          | 1996-11-05T00:00:00.000Z | FRIDAY
+10092          | 1989-09-22T00:00:00.000Z | FRIDAY
+10091          | 1992-11-18T00:00:00.000Z | SUNDAY
+;
+
+dayNameNull
+required_capability:fn_day_name
+from employees | where emp_no == 10040 | eval x = day_name(birth_date) | keep emp_no, birth_date, hire_date, x;
+
+emp_no:integer | birth_date:date                     | hire_date:date             | x:keyword
+10040          | null                                | 1993-02-14T00:00:00.000Z   | null  
+;