Some references used for this stub
- The close of the day before nightfall, when fog comes.
This is an approximative definition made up after the context of the poem. This looks like either an incredibly rare word not found in most dictionnaries, or merely a compound word poetically invented by Eliot ca. 1935, whose famous poem is also the earlier reference found online:
- The moment in the draughty church at smokefall
- -- T.S. ELIOT, in Four Quartets 1: "Burnt Norton" (1935) 
All other non-Eliot artistic occurences (googling excluding the poem ) are rare and later than Eliot, such as
- Keep safe from noonfall
- Starlight and smokefall where
- Waves roll, waves toll but feel
- None of our roving fever.
- -- Lawrence George DURRELL, in Selected Poems 1935-1963 (1964) 
- The largest sea ice perturbations are generated by smokefall in spring. - Ledley T. S., and S. L. Thompson (1986) "Potential effect of nuclear war smokefall on sea ice." (Climatic Change) HTML abstract and PDF article
- "animations and other images of the Cedar Fire incident", "The animation is from 29 October 2003, and shows a waterfall (smokefall) into the Anza Borrego Desert." 
- An artificial waterfall of smoke, for shows.