append fields for *where...* - Mailing list pgsql-sql

I have the following tables:

TABLE A
month   |   year   |   item         |   num
1              2005      myitem         003

TABLE B
num    |   date             |      descr
003         02-01-2005          blahblah
003         01-01-2005          toratora

I am trying to come up with something like:
select date,item,descr from TABLEA a 
LEFT OUTER JOIN TABLEB b ON  b.num=a.num
WHERE date=month-01-year;

How can you make month (append) - 01 - (append) year? and
pass that as a condition to the field date? Is this possible?