31 December:

The Luxembourg tax authority invites you, generally by letter, to prepare your annual tax return before 31 March.

However, this date is a recommendation and the actual deadline is 31 December of year n+1 (for income in year n).

After this date, the Luxembourg tax authority will no longer take any tax deduction into account. whether from special expenses, extraordinary expenses,…

31 March:

Applies for married taxpayers (resident or non-resident) willing to change their taxation method (e.g. switching from a joint to an individual taxation method or vice-versa). For these cases, the deadline is 31 March.