There are 2 types of "Lucky Investment" news: first event is when you can get the certificates and the second - when using the certs bring you extra rewards.
There was a Lucky Investment event on April 14-15th (
news post). If during this event you bought
50 (or more), you could receive one of the certificates (for example:
Certificate: Lucky Investment).
As the cert's description says, you can only use it in May 2021. So in May you can click on the cert to activate it, and then, once the cert is activated, purchase certain amout of diamonds within 24 hours to get the bonus (if you got the cert linked above, you need to purchase
50 to get additional
27).
The 2nd type of news is for an additional event, when activating the certs brings
extra rewards (for example (
this one).
So in short:
First, buy diamonds and get the cert. Next month use the cert and buy diamonds to receive the bonus. You can also wait for the "happy days" when you can get some extra rewards for activating the certificate.