Calculating the working hours in a given period - sql-server

Question

I'm trying to create a function where I can calculate the total working hours in a given period.
Explanation:
inputs are in mm/dd/yyyy hh:mm format.
For example user Inputs:
startDateTime: 01/10/2013 9:30am endDateTime: 12/12/2013 5pm
Daily working hours from 7am to 5pm. For example, if someone starts working at 9am and ends at 8pm them is working hours will calculate as 8 hours (only calculate the working hours between 7am to 5pm).
I do have a holiday list:
Every week's Saturday and Sunday are holiday
and bellow are the public holidays
HOLIDAY LIST:
07/04/yyyy
12/25/yyyy
12/31/yyyy
Memorial Day:- Last Monday of every May
Labor Day:- First Monday of Every September
Thanksgiving Day:- Fourth Thursday of every November
Now my problem is how to find out Memorial Day, Labor Day and Thanksgiving Day with in any given time period.
Final output will be the total working hours in the given period substituting the hours of holiday.
Bellow function I'm using only to calculate working hours eliminating weekends
CREATE FUNCTION [dbo].[WorkTime]
(
#StartDate DATETIME,
#FinishDate DATETIME
)
RETURNS BIGINT
AS
BEGIN
DECLARE #Temp BIGINT
SET #Temp=0
DECLARE #FirstDay DATE
SET #FirstDay = CONVERT(DATE, #StartDate, 112)
DECLARE #LastDay DATE
SET #LastDay = CONVERT(DATE, #FinishDate, 112)
DECLARE #StartTime TIME
SET #StartTime = CONVERT(TIME, #StartDate)
DECLARE #FinishTime TIME
SET #FinishTime = CONVERT(TIME, #FinishDate)
DECLARE #WorkStart TIME
SET #WorkStart = '07:00'
DECLARE #WorkFinish TIME
SET #WorkFinish = '17:00'
DECLARE #DailyWorkTime BIGINT
SET #DailyWorkTime = DATEDIFF(HOUR, #WorkStart, #WorkFinish)
IF (#StartTime<#WorkStart)
BEGIN
SET #StartTime = #WorkStart
END
IF (#FinishTime>#WorkFinish)
BEGIN
SET #FinishTime=#WorkFinish
END
DECLARE #CurrentDate DATE
SET #CurrentDate = #FirstDay
DECLARE #LastDate DATE
SET #LastDate = #LastDay
WHILE(#CurrentDate<=#LastDate)
BEGIN
IF (DATEPART(dw, #CurrentDate)!=1 AND DATEPART(dw, #CurrentDate)!=7)
BEGIN
IF (#CurrentDate!=#FirstDay) AND (#CurrentDate!=#LastDay)
BEGIN
SET #Temp = #Temp + #DailyWorkTime
END
--IF it starts at startdate and it finishes not this date find diff between work finish and start as minutes
ELSE IF (#CurrentDate=#FirstDay) AND (#CurrentDate!=#LastDay)
BEGIN
SET #Temp = #Temp + DATEDIFF(HOUR, #StartTime, #WorkFinish)
END
ELSE IF (#CurrentDate!=#FirstDay) AND (#CurrentDate=#LastDay)
BEGIN
SET #Temp = #Temp + DATEDIFF(HOUR, #WorkStart, #FinishTime)
END
--IF it starts and finishes in the same date
ELSE IF (#CurrentDate=#FirstDay) AND (#CurrentDate=#LastDay)
BEGIN
SET #Temp = DATEDIFF(HOUR, #StartTime, #FinishTime)
END
END
SET #CurrentDate = DATEADD(day, 1, #CurrentDate)
END
-- Return the result of the function
IF #Temp<0
BEGIN
SET #Temp=0
END
RETURN #Temp
END
Please help me how to find out the Thanksgiving Day, Labor Day and Memorial Day in a given period.