The publication date is the date on which a title is published. It is essentially your book's birthday and when it's officially available to the public.
The on-sale date (also called street date, street smart date, or release date) is the date upon which retailers are allowed to sell/deliver your book.
We suggest using the same date for the publication date and on-sale date when setting up your title with Lightning Source. This can help to avoid potential issues with the title availability messaging on some websites.
For more info about on-sale dates and setting up your title for pre-orders, please see here.