Q:

Is the CDC's reported drop today in COVID-19 cases by date of diagnosis meaningful?

I was waiting with bated breath for the CDC update on cases of COVID-19 in the U.S. by date of onset.  To my eyes this looks like absolutely wonderful news - that it means that the exponential growth of the virus has been "flattened" - but I'm still afraid I may be misinterpreting what I see.  It bothers me that the headline for the graph hasn't changed.  But it APPEARS that the number of new cases stopped increasing after 3/3.  Am I missing something?
Top: before noon on 3/16, bottom: after noon on 3/16.  Data from https://www.cdc.gov/coronavirus/2019-ncov/cases-in-us.html
Note: In today's update (the first in three days) the CDC page shows the number of cases with onset on 3/4 to be 159, and 3/9 is 307.  This doubled in five days rather than every two or three as other numbers would suggest, but given the dramatic changes in this curve, and their overall direction, I think the top answer below has been vindicated.

A:

The information you show is meaningless unless it is also accompanied by the number of tests performed. If the CDC tested 2 people and got two positive results, that doesn't tell us anything at all about the actual rate of infection. And, indeed, the CDC has been testing a shockingly low number of cases:

‡ Data during this period are incomplete because of the lag in time between when specimens are accessioned, testing is performed, and results are reported.

(image source)
The last day they give data for, 2020-03-15, includes 0 samples from the CDC and 4 samples from other public health labs. Since only 4 samples are represented in the chart, of course we see a drop in cases, but that's because they are based on only 4 tests. As more tests for those dates and subsequent dates are reported, the numbers will change.
Date Collected  CDC Labs    US Public Health Labs
3/12            75‡         2302‡
3/13            0‡          1511‡
3/14            0‡          165‡
3/15            0‡          4‡

As you can see in the table above (from the same source as the graph), the past few days have seen almost no tests are being included in this analysis either from the CDC or elsewhere, so any drop in cases is completely meaningless. 

