In the story is titled The Walls of Eryx. written in 1936, published in 1939 in Weird Tales Magazine, Lovecraft refers to the “mirage-plant” which from the description looked very much like cannabis and an incident under its inlfuences that happens at precisely 4:20:

“I had encountered at last one of those curious mirage-plants about which so many of our men told stories. Anderson had warned me of them, and described their appearance very closely – the shaggy stalk, the spiky leaves, and the mottled blossoms whose gaseous, dream-breeding exhalations penetrate every existing make of mask.”

And then Lovecraft goes on to have his main character have the following experience on the plant at precisely 4:20

“Although everything was spinning perilously, I tried to start in the right direction and hack my way ahead. My route must have been far from straight, for it seemed hours before I was free of the mirage-plant’s pervasive influence. Gradually the dancing lights began to disappear, and the shimmering spectral scenery began to assume the aspect of solidity. When I did get wholly clear I looked at my watch and was astonished to find the time was only 4:20. Though eternities had seemed to pass, the whole experience could have consumed little more than a half-hour.”

“Contemporary counterculture author and occultist Victor Cypert, on the website for the rock band Tool, has pointed to a passage from the story as a possible origin for the number “4:20″ being code for smoking marijuana”:

Reclaiming, Reprogramming, & Repossession”, by Victor T. Cypert, ToolNews, April 2002.” – http://en.wikipedia.org/wiki/In_the_Walls_of_Eryx