I realize that there is an appropriate time for a resignation letter. However, in a situation where you have not been offered a new yearly contract and you did not violate your contract in terms of paper work or duties and nobody is telling you anything then the Thai Administration's seemingly harmless sign this resignation letter document is not in your best interest because they will not have to pay you anything regarding severance pay.

Rights and duties of employers and employees under the new Labour Protection Act B.E.2541 http://www.thailabour.org/law/thai/code.html

Severance Pay

An employer shall pay severance pay to an employee whose employment is terminated, as follows:
An employee who has worked for at least 120 consecutive days, but for less than one year shall be paid basic pay for 30 days at the most recent rate of basic pay received by him.
An employee who has worked continuously for at least one year but less than three years shall be paid basic pay for 90 days at the most recent rate of basic pay received by him.
An employee who has worked consecutively for three years but less than six years shall be paid basic pay for 180 days at the most recent rate of his basic pay.
An employee who has worked consecutively for at least six years but less than 10 years shall be paid basic pay for not less than 240 days at the most recent rate of his basic pay.
An employee who has worked for more than 10 years consecutively shall be paid basic pay for not less than 300 days at the most recent rate of his basic pay.

<b>An employer is not required to pay severance pay to an employee whose employment has been terminated for any of the following reasons: </b>

<b>Resignation </b>
Dishonest performance of his duties or the intentional commission of a criminal act against the employer;
Intentionally causing loss to the employer;
Performance of gross negligence which result in severe loss to the employer;
Violation of the employer's work rules or regulations or order which are both lawful and equitable when the employer has already issued the employee with a prior written warning, except in a serious instance when the employer is not required to give a warning.
The written warning shall be effective for a period of one year as from the date of the commission of the violation by the employee;
Neglect of his duties for a period of three consecutive work days without reasonable cause, whether or not a holiday intervenes;
Imprisonment by reason of a final judgment.

An employment contract shall be terminated when the specified period in the employment contract expires, the works related are as follows:
8.1 Employment on a special project, which is not in the normal way of business or trade of the employer, where there is a fixed schedule for commencement and completion of work.
8.2 Work of a temporary nature with a fixed schedule for its commencement or completion.
8.3 Seasonal work in respect of which employees are only engaged during that season; provided that the work most be completed within a period of two years and the employer and employee have entered into a written agreement at or prior to the commencement of employment.