Midrasch zu Dewarim 31:11

(Exodus 23:14) "Three festivals shall you celebrate for Me in the year": What is the intent of this? Because it is written (Ibid. 17) "Three times, etc.", I might think at whatever place or time that one wishes; it is, therefore, written (Devarim 16:16) "on the festival of Matzoth, and the festival of Shavuoth, and the festival of Succoth." (Exodus 23:17) "shall be seen" (yod, resh, alef, heh, [which can also be read as "shall see"]) — to exclude the blind. "your males": to exclude women. "all your males": to exclude tumtum (one of indeterminate sex) and a hermaphrodite. (Devarim 31:11) "You shall read this Torah in the presence of all of Israel in their ears" — to exclude the deaf. (Ibid. 16:11) "And you shall rejoice" — to exclude one who is sick or a minor. (Ibid.) "before the L rd your G d" — to exclude one who is unclean — whence they ruled: All are obligated "to be seen" except a deaf-mute, a retard, a tumtum, a hermaphrodite, one who is blind, or sick, or aged.