Travelling from most locations, it is best to book your flight to Quito during the month of April for the best deal, although there are other times of the year which offer great deals as well.
In general, booking your departure to Quito for a Wednesday represents the best deal on airfare. Try to avoid a Saturday departure to stay away from the highest prices.
Generally, when visiting Quito, morning flights represent the best value. midday flights are likely to have higher prices.