What are the connotations of "Lo Sosifu ... , ve'Lo Sigre'u mimenu"?
Rashi and Ramban #1 (citing the Sifri): 'For example, not to write five Parshiyos in one's Tefilin, not to take five species with one's Lulav or to tie five Tzitziyos to one's garment'. 1 And the same applies to "ve'Lo Sigre'u".
Ramban #2: It also includes things like sitting in the Succah on Shemini Atzeres in order to fulfill a Mitzvah, 2 or keeping Succos for only six days.
Ramban #3: Someone even transgresses if he concocts a Yom-Tov not written in the Torah. 3
Ramban: Or even to add some words to Birchas Kohanim