If both columns are DATEs you can use =1
which means one day.
For Timestamps you need to tell what kind of interval you want:
WHERE (CONTRACT_EFFECTIVE_DATE - EVENT_TIMESTAMP) DAY = INTERVAL '1' DAY
But i'm not shure if this is what you really want, what's your definition of 1 day?
Edit:
Based on your comment the best way should be:
WHERE CAST(CONTRACT_EFFECTIVE_DATE AS DATE) - CAST(EVENT_TIMESTAMP AS DATE) = 1
This avoids dealing with INTERVAL arithmetic :-)