**Core Concept**
The Widal test is a serological test used to diagnose Typhoid fever, which is caused by Salmonella Typhi bacteria. The test detects the presence of antibodies against the O and H antigens of S. Typhi in the patient's blood. For the test to be reliable, it is essential to perform it at the right time to avoid false-negative results.

**Why the Correct Answer is Right**
The Widal test should be performed at least 2-3 weeks after the onset of symptoms when the antibodies have reached detectable levels. Performing the test too early, typically within the first week, may yield false-negative results due to the low levels of antibodies present in the blood. This is because the body takes time to produce a significant amount of antibodies against the S. Typhi antigens. The ideal time for performing the Widal test is when the patient is in the second or third week of illness, when the antibody levels are high enough to be detected by the test.

**Why Each Wrong Option is Incorrect**
**Option A:** Performing the Widal test too early, typically within the first week, may yield false-negative results due to low levels of antibodies present in the blood.
**Option B:** Performing the Widal test immediately after the onset of symptoms may not be reliable, as it takes time for the antibodies to reach detectable levels.
**Option C:** The Widal test can be performed at any time during the illness, but it is essential to wait until the antibodies have reached detectable levels to avoid false-negative results.
**Option D:** There is no specific time frame for performing the Widal test, and it can be done at any time during the illness.

**Clinical Pearl / High-Yield Fact**
It is crucial to note that the Widal test can be influenced by various factors, such as previous exposure to S. Typhi or other enteric pathogens, which can lead to false-positive results. Therefore, the test should be interpreted in conjunction with clinical findings and other diagnostic tests to ensure accurate diagnosis.

**Correct Answer:** C.