Soon is an adverb of indefinite time. Although it signifies time, it doesn't signify exact time. These adverbs can have three positions in a sentence:
At the end of the sentence: You will be promoted soon.
Before the verb (but between the auxiliary and the main verb): You will soon be promoted.
At the beginning of the sentence: Soon, you will be promoted. (This puts emphasis on 'soon'.)
All three are grammatical and the difference in meaning is more of a nuance: it is a matter of emphasis. I would say that perhaps 'soon' in the mid position puts emphasis on the verb, while front and end positions put emphasis on 'soon' (front position more so than end). An interesting article by June Casagrande discusses the positioning of the adverb 'soon'.
According to LEG, most adverbs can take front or end position, but not all can take mid position, like 'soon' in the 2nd sentence. For example, adverbs of definite time, such as tomorrow
- He will be promoted tomorrow. (1)
- Tomorrow, he will be promoted. (2)
Not: He will tomorrow be promoted. (3)
EDIT (inspired by comments): Although a pause in speech can make temporal expressions in mid position sound natural, this construction is not as common as the other two. (Since we are not interested in the exact phrase, but rather varying the position of a word/phrase it is difficult to offer proof - at least with my, somewhat limited, corpora searching skills. What I'm saying is: if you want to be on the safe side, don't correct people who use the third option, but when in doubt stick to the first two examples in your own sentences).
Please have in mind that the references I have cited are mostly intended for learners of English as a foreign language (as L.G. Alexander, the author of Longman English Grammar specifically states in the introduction). Therefore, some of the grammar concepts there are simplified and/or abridged. Also, there is a debate about whether dictionaries always categorise words accurately and there is a great reference about it provided in the comments below. The fact that some grammar references and dictionaries state that 'tomorrow' is an adverb doesn't mean that this is a rule etched in stone. This brings the discussion very far from your question, but if you would like more information, please see the comments below and this post on ELU.
Ref: LEG, CDO, edufind