It's one of those "must carry 'round" items you will have with you at all times.

Have the spectator think of any date in the year (his birthdate would be great). Toss him your little pocket calendar; "Open the calendar to your selected date, and as you do, note that I have hand written a different card in every day of the year; 365 days, seven decks of cards spread at random -- check it out.
"Do you have your date? If you had chosen the date before, would it have been the same card? No? What about the date after? No? For the first time, then, what card is written on your date?"
The spectator names the card.
The performer then removes an envelope from the same calendar held by the spectator and upon opening it, reveals one card and one card only... and it is the same card as named by the spectator!.

This is a diabolically clever effect and, more importantly, sufficiently strong to guarantee a lasting impression. It's always ready. This will be on your A List -- guaranteed.

BONUS - now including two great additional routines by two working professionals. You get an extrmely great effect and handling by Richard Osterlind and another by Richard Gerlitz. This plus Cassidy's original instructions give you a powerful impromptu killer effect.