It's fairly simple to do. Calculate the last day of the previous month, determine which day of the week it is, proceed x days to reach the day you want, then increment by given number of weeks.
dateCursor = DateSerial(DatePart("yyyy",dateCursor) + 1,10,31)
Do Until Weekday(dateCursor) = vbThursday
dateCursor = DateAdd("d", 1, dateCursor)
Loop
dateCursor = DateAdd("ww", 3, dateCursor)
dateCursor = DateSerial(DatePart("yyyy",dateCursor) + 1,1,31)
Do Until Weekday(dateCursor) = vbMonday
dateCursor = DateAdd("d", 1, dateCursor)
Loop
dateCursor = DateAdd("ww", 2, dateCursor)
Assuming you want to calculate the date during 2014, you would set the initial value of dateCursor to Now(). So to get Thanksgiving Day in 2014, try this:
<%
dateCursor = DateSerial(DatePart("yyyy",Now()) + 1,10,31)
Do Until Weekday(dateCursor) = vbThursday
dateCursor = DateAdd("d", 1, dateCursor)
Loop
dateCursor = DateAdd("ww", 3, dateCursor)
Response.Write "Thanksgiving Day: " & dateCursor
%>
. That says to add 1 year ("yyyy") to the current year. To get Thanksgiving for this year, change it to a 0; for 2015, make it a 2; etc.