First you have to understand what it is. Everyday is a good day to start an investment.
People's knowledge on bitcoin/cryptocurrency will always depend on how they understand the whole concept. As for me, I tell them about how I understand it and then refer them to a link that will back-up my explanation and for them to also explore and get to know the concept easier. There big media platforms that talks about cryptocurrencies. But the best for me is Ether. so for Ethereum, I will always suggest for people to check the website itself which is
www.propthereum.io All answers to possible questions is there and it has a very detailed whitepaper that can explain what it is.