from earlier in this thread...

Date: 06 Mar 05 - 03:26 AM

"I mark this day with a white stone," begins the April 25th entry in Charles Dodgson's personal journal for the year 1856. The white stone as marker is an archaic tradition of the Roman poet Catullus, signifying a particularly remarkable or wonderful day (Stoffel, 38).

and

The expression originates in Catullus' "Lapide candidiore diem notare," (Poem 68, line 148) which translates as "to mark with an especially white stone the (lucky) day." The English version was quite commonly used in Victorian times.