GringoPost | Ecuador: Quito airport departing flights

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Quito airport departing flights

In a perfect world, if I needed to be in Utah on September 20th, I could purchase a ticket departing Quito on the 19th.  But since there is a risk of being bumped, how many days prior to the 19th, should I plan on departing? 

That is, should I purchase a ticket for the 17th allowing for being bumped twice... Or maybe the 16th or 15th? I'll be flying United or Delta airlines.

Thank you,