Calculate date of easter based on Year passed. Based on Anonymous Gregorian Algorithm, also known as the Meeus/Jones/Butcher algorithm: https://en.wikipedia.org/wiki/Computus#Gauss_algorithm

find_easter_sunday.sql

CREATE OR REPLACE FUNCTION anonymous_easter(xyear integer) RETURNS date AS $$

DECLARE
  a int;
  b int;     
  c int;     
  d int;     
  e int; 
  f int;
  g int;
  h int;
  i int;
  k int;
  l int;   
  m int;          
  day_ int;  
  month_ int;

BEGIN
    a := mod(xyear, 19); 
    b := floor(xyear / 100);
    c := mod(xyear, 100);
    d := floor(b / 4);
    e := mod (b, 4);
    f := floor((b + 8) / 25);
    g := floor((b - f + 1)/3);
    h := mod ((19*a + b - d - g + 15), 30);
    i := floor(c / 4);
    k := mod (c, 4);
    l := mod((32 + 2*e + 2*i - h - k), 7);
    m := floor((a + 11*h + 22*l) / 451);
    month_ := floor((h + l - 7*m + 114) / 31);
    day_ = mod((h + l - 7*m + 114), 31) + 1;

   RETURN to_date(
    to_char(day_,    '00') || '.' ||
    to_char(month_,  '00') || '.' ||
    to_char(xyear,   '0000'),
   'DD.MM.YYYY'
   );
END;

$$ LANGUAGE plpgsql;

generate_italian_holidays.sql

CREATE OR REPLACE FUNCTION generate_italian_holidays("start_year" integer, "end_date" integer) RETURNS void

DECLARE 
  anno int;
  easter_date date;
BEGIN

CREATE TABLE holidays_calendar (
    holiday date NOT NULL,
    description character varying NOT NULL,
    id integer NOT NULL
);

FOR anno IN start_year..end_date
LOOP
     easter_date := planner.anonymous_easter(anno);
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-01-01', 'YYYY-MM-DD'), 'Capodanno');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-01-06', 'YYYY-MM-DD'), 'Epifania');
     IF (easter_date = to_date( anno ||'-04-25', 'YYYY-MM-DD')) THEN
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date, 'Pasqua e Festa della Liberazione');
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date + 1, 'Pasquetta');
     ELSIF (easter_date + 1  = to_date( anno ||'-04-25', 'YYYY-MM-DD')) THEN
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date, 'Pasqua');
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date + 1, 'Pasquetta e Festa della Liberazione');
     ELSE
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date, 'Pasqua');
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (easter_date + 1, 'Pasquetta');
       INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-04-25', 'YYYY-MM-DD'), 'Festa della Liberazione');
     END IF;
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-05-01', 'YYYY-MM-DD'), 'Festa dei lavoratori');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-06-02', 'YYYY-MM-DD'), 'Festa della Repubblica');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-08-15', 'YYYY-MM-DD'), 'Ferragosto');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-11-01', 'YYYY-MM-DD'), 'Ognissanti');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-12-08', 'YYYY-MM-DD'), 'Immacolata');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-12-25', 'YYYY-MM-DD'), 'Natale');
     INSERT INTO planner.holidays_calendar(holiday, description) VALUES (to_date( anno ||'-12-26', 'YYYY-MM-DD'), 'Santo Stefano');
END LOOP;
END;

$$ LANGUAGE plpgsql;

is_business_day.sql

-- Function: is_business_day(date)

-- DROP FUNCTION is_business_day(date);

CREATE OR REPLACE FUNCTION is_business_day(p_date date)
  RETURNS boolean AS
$BODY$DECLARE
    is_business_day boolean;
    dow int;
BEGIN
    dow := extract('dow' from p_date);

    IF dow = 6 OR dow = 0 OR EXISTS (SELECT 1 FROM holidays_calendar WHERE holiday = p_date)
        THEN
        is_business_day := FALSE;
    ELSE
        is_business_day := TRUE;
    END IF;

    RETURN is_business_day ;
END;$BODY$
  LANGUAGE plpgsql IMMUTABLE
  COST 100;
ALTER FUNCTION planner.is_business_day(date)
  OWNER TO support;
COMMENT ON FUNCTION planner.is_business_day(date) IS 'Returns true if day is IN(1,5) OR if is IN holidays_calendar';

working_date.sql

-- Function: planner.working_date(date, integer)

-- DROP FUNCTION planner.working_date(date, integer);

CREATE OR REPLACE FUNCTION planner.working_date(start_date date, duration integer)
  RETURNS date AS
$BODY$DECLARE 
    ret_date date;
    loop_date date;
BEGIN   
        --add days
        ret_date := start_date + (duration - 1);

        loop_date := start_date + 1; 
        --add extra day for each no business day between start_date and ret_date
        WHILE loop_date <= ret_date LOOP
            IF not planner.is_business_day(loop_date) THEN
                ret_date := ret_date + 1;
            END IF;
            loop_date := loop_date + 1;
        END LOOP;
        --add day if ret_date is no business day
        WHILE not planner.is_business_day(ret_date) LOOP
            ret_date := ret_date + 1;
        END LOOP;
     
        RETURN ret_date;
END;$BODY$
  LANGUAGE plpgsql VOLATILE SECURITY DEFINER
  COST 100;
ALTER FUNCTION planner.working_date(date, integer)
  OWNER TO support;
COMMENT ON FUNCTION planner.working_date(date, integer) IS 'Returns the corresponding end date based on a duration made only of working days';

To test the function over a time range you could use proficiently generate_series():

SELECT to_char(gday,'DD-MM') , is_business_day(gday::date) AS isbusiness
FROM generate_series('2016-12-01'::date, '2016-12-31'::date, '1 day') gday

In order to retrieve the result using Symfony and Doctrine you could use something like that:

  $em = $this->getDoctrine()->getManager();
  $query = $em->getConnection()->prepare('SELECT planner.working_date(:start_date::date, :duration::integer)');
  $query->bindValue(':start_date', '04/01/2017' );
  $query->bindValue(':duration', 100);
  $query->execute();
  $end_date = $query->fetchAll();