http://jesskinchen.com/finance/2016/02/15/spy-first-of-the-month.html

fom_spy.py

import csv

def is_first_of_month(row):
    date_string = row[0]
    return row[0].split('-')[2] == '01'

def handle_row(row):
    if is_first_of_month(row):
        open_price, close_price = float(row[1]), float(row[4])
        diff = close_price - open_price
        day_changes.append(diff / open_price)

day_changes = []
with open('table.csv') as csvfile:
    reader = csv.reader(csvfile)
    next(reader, None)
    for row in reader:
        handle_row(row)

average_change = sum(day_changes) / len(day_changes)
print("Average Day Change (percent): " + str(average_change * 100))