1) On the thirty third day of the Omer we increase happiness for the honor of Rabbi Shimon Bar Yochai who died on this day, and it is known that his will was that we should rejoice on this day. There are those who say that the rejoicing is because Rabbi Akiva ordained his five pupils (one of whom was Rabbi Shimon Bar Yochai) on this day. (Kaf HaChaim)
2) The thirty third day of the Omer is the day of rejoicing for Rabbi Shimon Bar Yochai, for when he died there was great rejoicing on high, when his soul ascended to heaven, and it is fitting for every man who has fear of heaven to concentrate on repenting on this day, for the merit of Rabbi Shimon Bar Yochai will help anyone who comes to purify himself. He should not waste his time with the emptiness of this world, which causes pain to the righteous man.