What publication lag is
An index for March does not come out in March. Statistical agencies need time to collect and check data, so the March value might publish in April, sometimes later. That delay is the publication lag, and it is different for every index and every country.
Why it causes disputes
If a contract says "apply the adjustment on the 1st of the month, using that month's index," and the index for that month has not been published yet, there is no value to apply. Both sides are left guessing whether to wait, use last month's value, or use a preliminary estimate that might still change.
Preliminary versus final values
Many indices publish a preliminary value first, then revise it once more data comes in. A clause that does not say which one to use invites an argument later, especially if the preliminary and final numbers differ enough to matter.
How to write around it
State three things in the clause: which specific release date is used for each adjustment date, whether the preliminary or final (revised) value applies, and what happens if the expected release is delayed. Naming the exact index vintage removes the argument before it starts.
The decision that actually matters
Pick an adjustment date that comes safely after the relevant index is normally published, not right on top of it. A one-month buffer between when the data is expected and when the adjustment takes effect avoids most publication-lag disputes without any other change to the clause.