[Leherenn]

Are you sure about the dates? As far as I know, you need to file your tax declaration for the previous fiscal year by mid-March, and pay before the end of March. But as you say, you're supposed to pre-pay most of it before based on an estimation (though you have to do the estimation yourself, not the government). You don't have to pay all at once though, you can pay in monthly installments with no penalties.