You receive a letter of dismissal. The first instinct, almost always, is to wait until you understand what happened, talk it over with people around you, and consult a lawyer a few weeks later. That is often a few weeks too many.
The Code of Obligations sets two separate deadlines. The first: a written objection must reach the employer before the notice period expires. If you are dismissed at the end of March with effect at the end of May, the objection must be sent before 31 May. It can run to three lines and does not need reasons, but without it everything that follows is closed off.
The second: court proceedings must be filed within 180 days of the end of the employment relationship. That period looks comfortable; it is used up quickly when negotiations drag on, and it is not extended merely because discussions were under way.
A dismissal is wrongful where it is given, among other reasons, on account of a personal characteristic, because of the exercise of a constitutional right, because the employee asserted claims under the contract in good faith, or in order to prevent claims from arising. Notice given during a protection period is void rather than merely wrongful: different mechanics, different calendar.
In practice, few of these files reach judgment. An objection filed on time, together with a letter setting out the facts soberly, is often enough to open a discussion about a proper employment reference and a severance payment. Provided the calendar has not already shut the door.
If a letter of dismissal is in front of you, note the two dates before anything else. Only then take the time to decide whether you want to act.