There are no rules about this, and in fact we could even
argue whether „(The) opening hours of the ski slopes“ is truly headline
style at all, or simply English with ellipsis (unnecessary words left out).
Ski slopes open Mon-Fri 11.00 – 16.00, Sat-Sun
0900 – 17.00.
„Ski slopes open“ is headline style, and it can be used where we know (or
can guess) from the context which ski slopes we're talking about – they're
the ones at this skiing resort. „(The) Opening hours“ is too prissy
(= overly fussy about detail) for true headline style.
The best way to get a feel for headline style is to read the headlines in
newspapers (published in a country where English is the first language), and
then read the text of the article to understand which elements of the
information the journalists chooses to flag up
and how they do that – which words they choose, and what kind of punctuation
(if any). In general, a good headline won't need any articles or
punctuation.