I searched the HOA documents for the 3 HGVC locations that I've owned and found it noted in the Declaration document.
The original declaration documents states that the checkin day of the week for SeaWorld is designated by the developer (see sample below). Flamingo and Lagoon Tower state that the day of the week is assigned by the board.
NOTE:The original Declaration document for each property may have changed in later amendments. I didn't check the various Declaration amendments.
Here's the section for Sea World (from the original declaration document)
2.45 Unit Week shall mean a period of ownership in a Unit committed to Component Site Timeshare Plan which shall consist of not less than seven (7) days commencing at 12:OO noon on a day of the week assigned by the Developer and ending at 12:OO noon on the same day of the week in the following week. Unit Weeks shall be numbered 1 through 52, and all Unit Weeks in a particular Unit shall begin and end on the same day of the week, although the beginning and ending day may vary from Unit to Unit. Unless otherwise designated by the Developer in the first deed conveying a Unit Week in a given Unit, the Unit Week shall begin and end on Saturday. Therefore, unless otherwise designated by the Developer, Week No. 1 is the seven (7) days commencing on the first Saturday in each calendar year, and Unit Week No. 2 is the seven (7) days immediately following. Additional weeks up to and including Unit Week No. 52 are computed in a like manner. Unit Week No. 52 contains the seven (7) days succeeding the end of Unit Week No. 51 without regard to the month or year. Any excess days not otherwise assigned shall be designated as Unit Week No. 53 and shall remain the property of the Developer, to be conveyed or retained as the Developer determines from time to time. References to "Unit Weeks" in this Declaration shall be deemed to include Biennial Unit Weeks unless the context dictates otherwise.