It is something that not only happens to everyone, but also is unpardonable: forgetting a birthday or important date. It just makes you feel bad.
To stop it happening again use Birthday Reminder, a program where you can save all the birthdays of your family, friends, contacts, clients, and everyone else you want to congratulate on their important day.
Birthday Reminder will advise you a few days beforehand and various times during the actual day. This can obviously work for other impoprtant days too, like anniversaries, dates, appointments, events, etc.
To work correctly you need to previously install
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NET Framework 2.0 or superior, and if you don´t have it, the program will detect the fact and install it for you.