When is it right to send an email when a conference misses its deadline?
It's fairly frequent, particularly for smaller or less tightly run computer science conferences, for the response to authors to slip back by a few days.  Often, this is because some reviewers may not have sent their reviews in on time, and if the program committee isn't proactive about scheduling for slippage and having backups available, it may take them a few extra days to get enough reviews to decide on those papers.  This then is likely to delay responses for all of the papers, because both the organizers and the conference software wants to sent all of the notifications at once.

So, to your question of what to do:


If you know anybody else who sent in a paper, compare notes with them.  If they haven't heard yet either, no worries.  If they have heard, then you can email immediately.
If you don't know somebody else, give it a week, and then ask.