Hello,

Please Check this code, 

from_date = fields.Date("From Date")
to_date = fields.Date("To Date")
month_days = fields.Selection([('Monday','Monday'),('Tuesday','Tuesday'),
                               ('Wednesday','Wednesday'),('Thursday','Thursday'),
                               ('Friday','Friday'),('Saturday','Saturday'),
                               ('Sunday','Sunday')], "Day")
day_count = fields.Integer("Number Of Days")


@api.onchange('from_date', 'to_date', 'month_days')
def onchange_count_days(self):
    count_days = 0
    if self.from_date and self.to_date and self.month_days:
        day_from = datetime.strptime(self.from_date, "%Y-%m-%d")
        day_to = datetime.strptime(self.to_date, "%Y-%m-%d")
        nb_of_days = (day_to - day_from).days + 1
        for day in range(0, nb_of_days):
            if day == 0 and str(day_from.strftime('%A')) == str(self.month_days):
                count_days += 1
            else:
                month_date = day_from + relativedelta(days=day)
                if str(month_date.strftime('%A')) == str(self.month_days):
                    count_days += 1
        self.day_count = count_days

Hope it will work for you,

Thanks