"100 days of happiness" is the closest English translation for "100天快乐". I'm not sure about the specific origins or context of this phrase, but here are some examples of how it could be used in English:

"I just got engaged! I'm looking forward to 100 days of happiness with my partner."

"My son just turned one year old. I can't believe it's been 100 days of happiness watching him grow and learn."

"I'm going on a solo trip to Europe for the first time. I'm excited for 100 days of happiness exploring new cities and cultures."

In each of these examples, the speaker is expressing their joy or excitement about a specific event or experience, and they are looking forward to a long period of happiness as a result.