Not all employment contracts may be terminated at earlier notice, whether by the employer or the employee. Because of that I am going to discuss each kind of the employment contract separately.
As a rule, an employment contract for a definite period of time may not be terminated at earlier notice before lapse of time for which it has been concluded (with some exceptions related to collective dismissals).
However, in relation to the employment contract to last longer than 6 months the parties may provide for a contractual clause authorizing either of them to terminate the contract earlier at two weeks’ notice. When this is the case, the contract may be terminated by the employer or the employee any time – it is mostly agreed that “any time” also means before the employee completes 6 months of employment (I used the word “mostly” here because it is the dominating view, also upheld by the Supreme Court, although there are also arguments against it).
Neither the employer nor the employee is required to provide reasons for giving notice. As a result, termination of the employment contract for a definite period of time is more like at-will rather than just-cause termination.