Without context, it's not clear what the sentence means. From the Wikipedia article for syntactic ambiguity:
Syntactic ambiguity, also called structural ambiguity, amphiboly or amphibology, is a situation where a sentence may be interpreted in more than one way due to ambiguous sentence structure.
There are some good examples in that article.
Usually the correct meaning is given by the surrounding text. In isolation, clearer wordings of the above could be:
- Read this letter. I wrote it in the evening
- Don't read my letter before the evening
Edit: remove comma splice