AttributeError: 'Series' object has no attribute 'isoweekday'

I wrote a function that moves the date by specified number of working days in respect to the weekends and holidays. It works fine when I use it with scalar values however I have issue trying to use on the pandas DataFrame. The function itself looks as follow:

def move_date_by_days(init_date, roll=1, nwd_key=None, hol_key=None):
    '''
    moves date by n-number of working days forward or backward
    init_date: date, initial caluclation date
    roll: integer, number of days to move forward (+) or backward (-)
    nwd_key: string that stands for currency iso code, it is a key in non_working_days dictionary
    hol_key: string that stands for currency iso code, it is a key in holidays dictonary
    return: date
    '''
    nwd = non_working_days.get(nwd_key, [])
    hol = holidays.get(hol_key,[])
    moved_date = init_date + datetime.timedelta(days=roll)
    if (moved_date.isoweekday() in nwd) or (moved_date in hol):
        if roll >= 0:
            moved_date=move_date_by_days(init_date + datetime.timedelta(days=1), roll=roll, nwd_key=nwd_key, hol_key=hol_key)
        else:
            moved_date=move_date_by_days(init_date + datetime.timedelta(days=-1), roll=roll, nwd_key=nwd_key, hol_key=hol_key)
    return moved_date

Now, I have DataFrame 'df' with the columns 'start_date' and 'end_date'

   start_date    end_date
0  2020-01-31  2020-04-30
1  2020-04-30  2020-07-31
2  2020-07-31  2020-10-31
3  2020-10-31  2020-11-28

and I would like to create the third column named 'fixing' that would be 2 working days before 'start_date'. I am trying this:

dates_table['fixing'] = move_date_by_days(self.dates_table['start_date'], -2, self.ccy, self.ccy)

but it returns AttributeError: 'Series' object has no attribute 'isoweekday'

Please note the function works fine when I refer to scalar data of the DataFrame, for instance

d1 = s1.dates_table.iat[0,0]
move_date_by_days(d1, -2, 'pln', 'pln')

it returns what I have expected: datetime.date(2020, 1, 29)

Any hint how could I use the function on the whole column of the data frame, please?