However, it could also be the reaction if your company is approaching or is currently in a busy season and needs the additional staff, or it's experiencing unusual circumstances, like a hiring freeze, which would prevent it from replacing you entirely.
Your End Date. The most crucial detail in a resignation letter is your end date. Legally and according to your company's policy, you might be required to give a minimum amount of notice (two weeks, or possibly more, depending on the company and the role). If that's the case, you don't want there to be any confusion about how much notice you give. If it's right there in writing in a letter dated two weeks before your last day, you're saving yourself the hassle.