days.py

# -*- coding: UTF-8 -*-

DAYS_OF_THE_WEEK = {
    "en_US": {"Sunday":  0, "Monday":  1, "Tuesday": 2, "Wednesday": 3, "Thursday": 4, "Friday":  5, "Saturday": 6},
    "es_ES": {"Domingo": 0, "Lunes":   1, "Martes":  2, "Miercoles": 3, "Jueves":   4, "Viernes": 5, "Sabado":   6},
    "ja_JP": {"日曜日":  0,  "月曜日": 1, "火曜日":  2, "水曜日":    3,  "木曜日":  4,  "金曜日": 5, "土曜日":   6},
}

REVERSE_DAYS_OF_THE_WEEK = dict((locale, dict(map(reversed, days.items())))
    for locale, days in DAYS_OF_THE_WEEK.items())

def day_offset(day_name, day_count, locale):
    day_number = (DAYS_OF_THE_WEEK[locale][day_name] + day_count) % len(DAYS_OF_THE_WEEK[locale])
    return REVERSE_DAYS_OF_THE_WEEK[locale][day_number]

# Test cases:
assert day_offset("Sunday", 0, "en_US") == "Sunday"
assert day_offset("Sunday", 1, "en_US") == "Monday"
assert day_offset("Sunday", 2, "en_US") == "Tuesday"
assert day_offset("Sunday", 6, "en_US") == "Saturday"
assert day_offset("Sunday", 7, "en_US") == "Sunday"
assert day_offset("Sunday", 8, "en_US") == "Monday"
assert day_offset("Wednesday", 4, "en_US") == "Sunday"
assert day_offset("Wednesday", 14, "en_US") == "Wednesday"
assert day_offset("Wednesday", 15, "en_US") == "Thursday"
assert day_offset("Saturday", 15, "en_US") == "Sunday"
assert day_offset("Saturday", -1, "en_US") == "Friday"
assert day_offset("Saturday", -14, "en_US") == "Saturday"
assert day_offset("Saturday", -16, "en_US") == "Thursday"
assert day_offset("Lunes", 3, "es_ES") == "Jueves"
assert day_offset("Lunes", 8, "es_ES") == "Martes"
assert day_offset("Lunes", -8, "es_ES") == "Domingo"
assert day_offset("Lunes", 0, "es_ES") == "Lunes"
assert day_offset("Lunes", 0, "es_ES") == "Lunes"
assert day_offset("日曜日", 0, "ja_JP") == "日曜日"
assert day_offset("日曜日", 2, "ja_JP") == "火曜日"