Great Britain & Wales
Republic of Ireland
Note! if you are here to find the dates of specific holidays because you searched for "national holidays", "federal holidays" or "bank holidays", you can use this site to view the dates of holidays for a specific year.
MSDN has great articles on how to consume web services.
Using Visual Studio.NET:
Without using Visual Studio.NET, from ASP.NET:
Without using Visual Studio.NET, from a console application:
View the ASP Code - note, this is for the v1 services, but the principals are the same. The v2 services should be easier because there isn't the .NET Dataset guck to deal with.