[pdftex] circled and boxed digits?

[Allin Cottrell]

I'm preparing an annual calendar, where three sorts of significant 
dates need to be identified, in such a way that the identification 
will be clear whether the calendar is printed in colour or 
greyscale.

I thought of marking the special dates in this way:

Type 1: put the date (day of month) inside a little red circle
Type 2: date inside a blue rectangle
Type 3: date inside a green diamond

My question is: what's the most straightforward way of implementing 
this sort of trickery using pdflatex?  (The circles, boxes, etc. 
should, ideally, not disrupt the usual tabular arrangement of the 
dates in the calendar -- they should act as a pure overlay.)

-- 
Allin Cottrell
Department of Economics
Wake Forest University, NC