First Day of the YEAR : ROUND (DATE)

First Day of the YEAR by ROUND function

This function works similar to ROUND of numbers, if the month is geater than six (6) then return the day of next year else it returns day of current year.

SELECT ROUND (SYSDATE, 'YEAR') "As of day" FROM DUAL;
/

SELECT ROUND (TO_DATE ('12-mar-2021'), 'YEAR') "Current Year" FROM DUAL;
/

SELECT ROUND (TO_DATE ('12-aug-2021'), 'YEAR') "New Year" FROM DUAL;
/

First and Last Day of the year

SELECT TRUNC (SYSDATE, 'YEAR') "First Day" FROM DUAL;

SELECT ADD_MONTHS (TRUNC (SYSDATE, 'YEAR'), 12) - 1 "Last Day" FROM DUAL;

First and last Day of Month

SELECT TRUNC (SYSDATE) - (TO_NUMBER (TO_CHAR (SYSDATE, 'DD')) - 1) FIRST_DAY FROM DUAL;

SELECT   ADD_MONTHS (
            TRUNC (SYSDATE) - (TO_NUMBER (TO_CHAR (SYSDATE, 'DD')) - 1),
            1)
       - 1 LAST_DAY
  FROM DUAL;

OR

 SELECT TRUNC(LAST_DAY(ADD_MONTHS(SYSDATE,-1)))+1,
      TRUNC(LAST_DAY(SYSDATE)) 
 FROM DUAL;

OR

 SELECT TRUNC(LAST_DAY(SYSDATE)) LAST_D FROM DUAL;