Two week notice or longer?

Should I give more than a two week notice if a holiday (ie. thanksgiving and the day after) falls in the middle of the two weeks time period?

Additionally, is it courteous or unnecessary to give three weeks notice instead of two? In the last couple weeks, there have been a few employees who have resigned or been fired from my small company; we are understaffed even more so than usual. I want to leave on good terms but also know my departure will cause some disarray.

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