If you are liable to make French Income Tax Returns (even if you have no tax to pay) they have to be submitted by midnight on 31st May.
If you want to submit via the internet (and it is really no more difficult than doing it on paper) you have extended deadlines dependent on the number of the department in which you live. For 81 and 82 it is 21st June for Aveyron, dept 12, it is 7 June 2012. The government site below is where to start.
www.impots.gouv.fr
