The phrase loading days came about after Dr. A.T.W. Simeons developed the HCG diet plan in 1954, but it is more than descriptive.
In his seminal work Pounds and Inches, Dr. Simeons put it this way:
Patients in a satisfactory general condition and those who have not just previously restricted their diet start forced feeding on the day of the first injection. Some patents say that they can no longer overeat because their stomach has shrunk after years of restrictions. While we know that no stomach ever shrinks, we compromise by insisting that they eat frequently of highly concentrated foods such as milk chocolate, pastries with whipped cream sugar, fried meats particularly pork, eggs and bacon, mayonnaise, bread with thick butter and jam, etc.
We no longer recommend costly, painful injections because HCG diet drops are just as effective. However, the start of the HCG diet plan is exactly the same as Dr. Simeons envisioned it—eat to your heart's delight.
On the third day, the start of Phase 2, you will then be ready to restrict yourself to 500 calories a day while avoiding anything with starch or sugar. You'll be ready because the metabolic effects of the HCG diet drops will have kicked in by then.
So for the first two days on the official HCG diet plan, load up. Enjoy your loading days while the HCG drops make you a new person.