The EPO has so many publication dates for each patent: A1, A2, A3, A4. . . B1, B2, etc.
Which should I use as publication date?
You should use the date of the A1 or A2 publication for the earliest publication date of the subject-matter (of the application).
(If you want to know about the granted patent please clearify.)
A1 is a publication of the application with search report (SR).
A2 is the publication without SR. There will be always only one or the other.
An A3 is published later in case of an A2 as soon as the search has been carried out. See here for more details on the so called kind codes: https://register.epo.org/helphttps://register.epo.org/help?lng=en&topic=kindcodes
18 months after application an application is published by the EPO according to Article 93(1)(a) EPC:
"The European Patent Office shall publish the European patent application as soon as possible
(a) after the expiry of a period of eighteen months from the date of filing or, if priority has been claimed, from the date of priority"
, if not requested earlier.
Always the first one. The follow ups are for some admin or legal back logs.