This glitch also was documented by Edgardas Balsevičius, who shows in the video below that his iPad is set to Lithuania for its region, but the default holiday calendar is set to other countries, including Mexico, Hong Kong, and Canada. This mismatch between location and calendar is not observed when the region is set to the USA, UK or Russia.
This glitch was reported over the weekend and is present on iOS devices, including an iPhone 5s and an iPad Air, running iOS 7.1.x. Apple support has allegedly confirmed to Balsevičius that this is a known software issue that will be addressed in the next update.
While most reports involve iOS 7.1, a few OS X owners have reported a similar problem with OS 10.9.x that also started in the past week. These users also state that the default holiday calendar now displays events from the incorrect region, with one user on Apple Support forums noting that switching the default currency back to the local designation seems to fix the issue.