Hi,
Which of the following do you find more natural:
“Today was not a very effective day. I sold only 10 products”
“Today was not a very efficient day. I sold only 10 products”
Thank you very much.
I think both possible. It might depend on what you would like to say
This is definitions of those words from the google dictionary.
effective “Something that is effective works well and produces the results that were intended.”
efficient “If something or someone is efficient, they are able to do tasks successfully, without wasting time or energy.”
Please look at the those phrases “that were intended” and “without wasting time or energy”.
In addition, Efficiency in economics is the status that marginal cost and marginal revenue are same.
Thank you very much, Chang. I would also like to know ideas of some native speakers 
Regards,
I wouldn’t find either of them as natural as
“Today was not a very successful day…”
Of the two, ‘efficient’ is better, but it’s not ideal.