It's related to holy. Probably this meaning arose because Saturday and Sunday are considered holy days in Christianity or alternatively, because both the Norse "helg" and the English "holy" are ultimately derived from an ancient stem that meant whole, complete, i.e. "helg" is the part of the week which completes it.
Yes. Those recordings are dialect. There are simular recordings of “elg” (moose), with the “j-sound”. “G-sound” is the safe one here, unless one is planning on learning a specific dialect. Another example of a “ge” with differing pronunciation is “gevær” (rifle). The standard is “g-sound” while some will pronounce it with a “j-sound”.
The main rule seems to be that original (Nordic) words have a “j”-sound in front of “y”, “i”, “ei” and partly in front of “e”. The exceptions are mainly the imported words. There are also exceptions to the rule of the original Nordic words. Finally, there are exceptions to the exceptions (the imported words). “Old imports” may have been adopted. “Gir” as an example - The rule: “jeg gir” (I give): The verb "gi" from Norrønt. "j"-sound - The imported exception: "gir" (gear): From English. “g”-sound - The exception to the exception: "gir" (similar to the idiom “give way”?): From Dutch / German. “j”-sound “G” can also have a “sj”-sound: gele / gelé (pron: sjele) and giro (pron: sjiro) http://riksmalsforbundet.no/qa_faqs/uttale-av-g-foran