This is mentioned by his father, the Maharil, in order to bring out that although on Shabbos Chazon according to halacha someone is not supposed to put on his Shabbos clothing. In case of a Shabbos Spinholtz–Aufruf, the chassan and his father are exceptions. The father is allowed to wear his Shabbos cloak (overcoat/rezhvolka), and the chassan is allowed to put on all his Shabbos clothes.
Source: Maharil, Minhagim; date and location based on I.J. Yuval, Scholars in their time, page 226, footnote 5.
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