Find the 1st business day and last business day of the previous month based on a date

Question

I have a use case like this in Python: Find the 1st business day and last business day of the previous month based on a date. For example if date is 2024-06-10

first_business_day = '2024-05-01'
last_business_day = '2024-05-31'

I have tried like below

run_date = '2024-06-10'
from datetime import datetime, timedelta

d = datetime.strptime(run_date, '%Y-%m-%d').date()
previous_month_first_business_day = (d - timedelta(days=d.day)).replace(day=1).strftime("%Y-%m-%d")
previous_month_last_business_day = (d - timedelta(days=d.day)).strftime("%Y-%m-%d")

Result:

previous_month_first_business_day = '2024-05-01'
previous_month_last_business_day = '2024-05-31'

This is working fine for month of May, but when I want the same result for June, then using the above I am getting:

previous_month_first_business_day = '2024-06-01' # This should be '2024-06-03'
previous_month_last_business_day = '2024-06-30' # This should be '2024-06-29'

What should I do to achieve the right result?

Answer 1

Take a look at the builtin calendar instead of just datetime

import calendar
import datetime

def month_start_end_work(date_src, fmt_date="%Y-%m-%d"):
    dt = datetime.datetime.strptime(date_src, fmt_date)

    # roll around January -> December
    month = 12 if dt.month == 1 else (dt.month - 1)
    year = dt.year if month != 12 else (dt.year - 1)

    # discover last day of last month
    for day in (31, 30, 29, 28):  # handle Feb cases
        try:  # already the correct year for leap
            month_start = calendar.weekday(year, month, day)
        except ValueError:  # day is out of range for month
            continue  # month has fewer days (always decreases)
        if month_start == calendar.SATURDAY:
            day -= 1
        elif month_start == calendar.SUNDAY:
            day -= 2
        break  # last day name to be used
    else:  # did not solve and break
        raise RuntimeError("BUG: impossible code path reached")
    day_last = (year, month, day)  # TODO may want to make a datetime.datetime
    
    # discover first day of last month
    day = 1  # all months begin at 1
    month_start = calendar.weekday(year, month, day)
    if month_start == calendar.SATURDAY:
        day += 2
    elif month_start == calendar.SUNDAY:
        day += 1
    day_first = (year, month, day)

    return day_first, day_last

>>> month_start_end_work("2024-06-10")
((2024, 5, 1), (2024, 5, 31))
>>> month_start_end_work("2024-07-10")
((2024, 6, 3), (2024, 6, 28))
>>> month_start_end_work("2022-01-01")
((2021, 12, 1), (2021, 12, 31))

If you have special days you know are days off (probably 6-12 annually), collect them into a list and just keep decrementing/incrementing for exact matches