n. — Outdoors
a sunny day during a period of poor weather, sometimes thought to precede a storm.
Type: 2. Preservation — Pet day is a preservation from Scottish or Irish English. OED-3 lists the term as originally Scottish (see OED-3, s.v. "pet day"), however EDD lists it as Irish (see EDD s.v. "pet" (2)), and it is found in DHE and described as "Galway", although with "origin obscure" (see DHE, s.v. "pet day").
ITP Nelson lists the term as "Prince Edward Island" (see ITP-Nelson, s.v. "pet day"), but quotations suggest that it has currency across much of the Maritimes, including New Brunswick (see, e.g. the 2002 quotation), Newfoundland (the 1998 quotation), Nova Scotia (the 1891 quotation), and the Magdalen Islands (the 2006 quotation).
A slight semantic shift may have taken place in the Canadian usage. EDD, DHE and OED-3 define the term in similar ways as a sunny day during a period of otherwise poor weather, while in DPEIE, pet day denotes a fine day that is thought to precede a storm. DNE defines the term more like OED-3, but the quotations under the headword include the DPEIE meaning (see DNE, s.v. "pet").