On Wikipedia, as in the other projects here, I assume, correct punctuation is important.
Per [[WP:Copyedit#Punctuation]], two commas are missing in the following current message:
Wikimedia technical Operations is planning a major infrastructure migration on Wednesday, 12 September and Wednesday, 10 October 2018 at 14:00 UTC.
There should be one comma after "12 September", and one after "10 October 2018". Without the commas, the sentence falls into three parts, the second of which is dangling:
- Wikimedia Technical Operations is planning a major infrastructure migration on Wednesday
- 12 September and Wednesday
- 10 October 2018 at 14:00 UTC
"2018" is probably superfluous.