Lag B'Omer is the 33rd day of the Omer. the word "lag" derives from the Hebrew letters used to spell the number 33. Lamed = 30 and gimmel = 3, thus ל"ג is 33.
Zohar Idra Zuta Devarim, Haazinu; Death/Release from Cave of Rabbi Shimon Bar Yochai attributed to Lag B'Omer:
On the day that Rabbi Shimon bar Yochai was to leave this world he arranged his affairs. His friends came to his room and he said to them “now is a time of favour. I can now reveal to you holy things that haven’t been revealed until now”…. All that day the fire never left the room, and there was nobody who was able to approach because it was impossible. The light and the fire were surrounding him. When they [came to] remove his bier the fire flew into the air and the fire danced before it. A voice was heard [from Heaven] saying “come and gather for the hilula (anniversary/celebration) of Rabbi Shimon bar Yochai”.
Midrash Tanhuma on Genesis 1
The Torah is full of holy fire; it was written with a black fire upon a white fire.
and, as My Presence passes by, I will put you in a cleft of the rock and shield you with My hand until I have passed by.
(29) Behold, My word is like fire—declares the LORD—and like a hammer that shatters rock!
Jerusalem Talmud, Tractate Shekalim 6:1, 25b
Babylonian Talmud, Tractate Hagigah 27a
Midrash Tanhuma
Please teach us rabbi, if a fire occurs on the Sabbath in a house in which there is a Torah scroll and other (holy) books, can one save them from the fire? [Answer] This is what our rabbis have taught: All holy writings should be saved from a fire on the Sabbath, whether or not they are read [as part of the liturgy] (Tractate Shabbat 16:1). Why did they say they should be saved? This is owing to the honor of the Torah that is written upon them, for if they were to be burned, they would become worthless. You can find that when the Holy One, blessed be He, gave the Torah, everything consisted of fire, as it is said: From his right a fiery law to them (Deuteronomy 33:2). Reish Lakish said: “The Torah was of fire; its hides were fire; its writing was fire; its sewing was fire.”….The intermediary’s face became fire, as it is said: Aaron and the Israelites saw that the skin of Moses’ face was radiant and they shrank from coming near him (Exodus 34:30)….The mountain was ablaze with flames (Deuteronomy 4:11) and it (the Torah) was given from fire that consumes fire as it is said: For the Lord your G!d is a consuming fire (Deuteronomy 4:24)
Rav Abraham Isaac Kook, 1865–1935, first chief Ashkenazic rabbi of British Palestine
Black Ink on White Parchment
When we think about a Torah scroll, we usually only consider the letters themselves, written in black ink. Yet, the Talmud (Menachot 29a) rules that every letter in a Torah scroll must be completely surrounded by parchment. This requirement is called mukaf gevil. In other words, the white parchment around the letters is an integral part of the Torah; without it, the Torah scroll is disqualified. In fact, the white space is a higher form of Torah. It is analogous to the white fire of Sinai — a sublime, hidden Torah that cannot be read in the usual manner.
There is a delicate balance between black and white in the Torah. The shirot, the poetic portions in the Torah, are written in a special fashion, like a wall constructed from layers of black and white bricks. These poetic sections are the loftiest parts of the Torah. Consequently, they have more white space, as they contain a greater measure of the esoteric white fire. If a scribe were to write other sections of the Torah in this special layout, the Torah scroll would be rendered invalid. After the Torah was revealed and restricted to our limited world, it must be written with the appropriate ratio of black to white.
The Divine Call Before Revelation
The distinction between white and black fire also sheds light on God's call to Moses before speaking with him. The Voice summoning Moses to enter the tent was in fact the divine call from Sinai, an infinite call that never ceased (Deut. 5:19).…This Voice was not a revelation of Torah, but an overture to its revelation. It belonged to the esoteric white fire of Torah, before its constriction and revelation into the physical world.
This is the reason that Moses made the aleph of the divine call smaller. Since it belonged to the realm of white fire, the summons required an extra measure of white space over black ink. Superficially, Moses' miniature aleph humbly implies a diminished state of the revealed Torah of black fire, but on a deeper level, it reflects an increase in the esoteric Torah of white fire.
-Both passages by Rav Kook from Gold from the Land of Israel, pp. 179-181.
Rabbi Avi Weiss, 1944 to present, American Modern Orthodox
On the simplest level, black fire refers to the letters of Torah, the actual words, which are written in the scroll. The white refers to the spaces between the letters. Together the black letters and white spaces between them constitute the "whole" of the Torah.
On another level, the black fire represents the p'shat, the literal meaning of the text. The rabbis point to the importance of p'shat when stating "the text cannot be taken out of its literal meaning." The white fire, however, represents ideas that go beyond the p'shat. It refers to ideas that we bring into the text when we interact with it. This is called d'rash-interpretations, applications, and teachings that flow from the Torah. The d'rash are the messages we read between the lines.
On yet another level, the black letters represent thoughts which are intellectual in nature, whether p'shat or d'rash. The white spaces, on the other hand, represent that which goes beyond the world of the intellect. The black letters are limited, limiting and fixed. The white spaces catapult us into the realm of the limitless and the ever-changing, ever-growing. They are the story, the song, the silence. Sometimes I wonder which speaks more powerfully, the black, rationalistic letters or the white, mystical spaces between them.
-Excerpt from http://www.hir.org/a_weekly_gallery/8.16.02-weekly.html