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The 3 month maximum lead time for a Schengen application is statutory. The statement occurs in the Visa Code, but it is more accessible in the summary page at EUR-Lex
A visa application may be lodged by the applicant or an accredited commercial intermediary at the earliest 3 months before the intended visit. When lodging an application, the applicant must appear in person, unless this requirement has been waived. When lodging an application, the following must be presented:
(emphasis mine)
Note: it does not matter if the 'three-month forward computed date' falls on a weekend or holiday.