[Fr. de Sivry] wore red because that day, [August 9,] was the feast of the Vigil of St Lawrence.
How could I doubt that the above explanation was written by 'poche'?
Ah, yes. The "
feast of a vigil"! I'd wonder if that weren't yet another innovation from your
Novus Ordo, except that your
Novus Ordo doesn't have
vigils--at least not in the
traditional sense. Not even for Christmas, Easter, Ascension, nor Pentecost. The first one has been reduced to a
weekday "of the 4th Week of Advent"; for the latter 2, what would traditionally be
vigils have been reduced to mere
weekdays of an ordinally numbered "Week of Easter". It's your
Novus Ordo that's notorious (among variously leaning traditional Catholics) for its current application of the word
vigil to the Saturday-afternoon weekly masses beloved by fans of the Sunday-game-intensive (U.S.) National Football League.
Assuming that I can trust Kenneth G. Bath's
Novus Ordo-only liturgical-calendar programming for RomCal.com, August 9 is the "Opt. Mem." of "
Teresiae Benedicta of the Cross, V & M", for whom the liturgical color is
green.
In my compilation for the calendar of 1960 (from a discarded mid1960s
Ordo), which I would assume SSPX relies upon, August 9 is the III-class feast of "
S. Ioann M. Vianney, C.", for whom the liturgical color is "
alb.", i.e.:
white!
But SSPX disagrees: August "
9 Our Lady's Saturday", "4cl[ass]", which shows the liturgical color as a surprisingly yellowish off-white, but a shade of
white nevertheless. For the patron-saint of priests, it shows August "
8: St. John Mary Vianney", 3cl[ass]".[
S] 
Lastly, there's Tr@ditio's "Calendar for the Traditional Roman Rite", August "
9 Saturday[:] St. John Mary Vianney, C", for whose feast of the
double minor class (i.e.: 4th rank), the liturgical color is
white. Its calendar for that date does show, as alternatives of distinctly lesser priority, the "
Vigil of St. Lawrence" (but see "violet", below), plus what I assume is at most a
commemoration of "St. Romanus, M".
St Lawrence was a martyr. Red is the color of martyrs.
Indeed it is, but only on their
feast days. Even if the date under discussion in the SSPX calendar were primarily assigned to a vigil, I believe that an inspection of any traditional or semitraditional liturgical calendar will show that the liturgical color for any
vigil is
violet (e.g.: August 14: Vigil of the Assumption)--
except for the traditional dual violet-then-white colors on Holy Saturday.
Can any of the former seminarians reading CathInfo point out anything liturgical that I might've overlooked?
-------
Note #: E.g.: <
romcal.com/output/2014.html>.
Note [
S]: <
www.fsspx.org/en/liturgy/the-liturgy/liturgical-calendar/calendar/?month=aug&yr=2014>
(an artistic Web
page, but an obnoxiously long Web
address, what with the latter containing 3 combined instances of "liturgy" or "liturgical", and 2 instances of "calendar").
Note *: <
www.traditio.com/calendar/cal1408.htm>.