There are two minhogim about Behab,
1 minhag is the first monday after the first shabbos after rosh chodesh cheshvan.
the 2nd minhag is, the 3rd monday after rosh chodesh cheshvan, to mirror the taaniyos mentioned in the mishna in taanis, when there was no rain.
The 2nd minhag is generally the minhag ashkenaz (aka yekkishe minhag) and also results in a shorter taanis (in the n. hemisphere).