Here I excerpt two useful sections in Chapter 12 of the book Option Volatility & Pricing.
Choosing an Appropriate Strategy
If implied volatility is low, such that options generally appear underpriced, look for spreads with a positive vega. If implied volatility is high, such that options generally appear overpriced, look for spreads with a negative vega.
Long calendar spreads are likely to be profitable when implied volatility is low but is expected to rise; short calendar spreads are likely to be profitable when implied volatility is high but is expected to fall.