Most contractions are just simple elisions in French (one or two final letters are replaced by an apostrophe). Only four are non-trivial, each of them involve one preposition and one article (they don't apply when le or les are pronouns):
- au = “à le”
- aux = “à les”
- du = “de le”
- des = “de les”
Those contractions are always performed, except when the article le, which is part of these compounds, can be elided first (example: de l'abandon, à l'abandon). This is actually the only point you should be careful about.
Now, because au, à or aux begin with a vowel, the word that precedes might also require an elision (but this was expected):
Jusqu'aux premières lueurs du soleil.
In very rare situations, several elisions could be chained.