The holy grail is treasure that comes from a religious background. Feel free to search for the Holy Grail on Wikepedia and you will understand its importance. In Arabic we have the equivalent "الكأس المقدسة" but this doesn't go with your context here.

It is said to be of miraculous powers or benefits of the same sort and that's why it is so important and everybody who knows about want to put his hand on it.

There are many movies stories where the Holy Grail was present such as giving an eternal life for whoever drinks from the Holy Grail and other stuff like that...

Now in the context of your question, it is referring to something very important and eveyone is highly interested in it (great diet and a stress-free holiday).

Good luck!

يجب مراعاة لمن النص موجه، فإذا كان موجه لمسيحيين فيمكن أن نقول "الكأس المقدس" لأن هذا رمز يفهم المسيحيون قصته، أما إن لم يكن ذلك فيجب البحث عن مقابل ثقافي أو حتى تعبيري ومعنى الكلمة في هذا السياق هو "Definition of Holy Grail. : something that you want very much but that is very hard to get or achieve". فأقترح أن نقول الكنز المفقود الذي يبحث عنه الناس أو ممكن فتاة أحلام الناس أو أملهم المنشود

The term "holy grail" is often used to denote an elusive object or goal that is sought after for its great significance.