Remember March, the ides of March remember:
Did not great Julius bleed for justice’ sake?
What villain touch’d his body, that did stab,
And not for justice? What, shall one of us
That struck the foremost man of all this world
But for supporting robbers, shall we now
Contaminate our fingers with base bribes,
And sell the mighty space of our large honours
For so much trash as may be grasped thus?
I had rather be a dog, and bay the moon,
Than such a Roman.

If you want to know more about the ancient Roman calendar, you can check out this page. The calends of a month (kalendae) were the first day of a month, thought to have originally been the day of the new moon. The ides of a month (idus) were thought to originally have referred to the day of the full moon. That was the 13th in 29-day months, but the 15th in 31-day months (which were only March, May, July, and October). The name of the month appears as an adjective agreeing with the day; idus is feminine, so the day is called idus Martiae, or, in the expression, cave idus Martias.

No, Cassius, no: think not, thou noble Roman,
That ever Brutus will go bound to Rome;
He bears too great a mind. But this same day
Must end that work the ides of March begun;
And whether we shall meet again I know not.
Therefore our everlasting farewell take:
For ever, and for ever, farewell, Cassius!
If we do meet again, why, we shall smile;
If not, why then, this parting was well made.

Also, how about merging this with Pi Day and having the Pides of March?