The art and science of selecting an auspicious time is called Muhrata in Vedic astrology. You may argue that every moment belongs to God (or Universe or Kala Purusha i.e. eternal cosmic being), then how can one instance of time be better or worse than the other?
Pure Science: If you ever rode a bicycle, you will know that riding against the wind takes much more effort than riding with the wind. Similarly, sailing against the wind is not as easy as sailing with the wind. And sowing a seed during peak winter most probably will not yield a good crop versus sowing during the spring or appropriate season. Similarly, tasks, projects or journeys initiated at the “auspicious times” have more chances of yielding better results.
Does that mean guaranteed success? Of course not. There is no guarantee that selecting an auspicious time will remove all obstacles. Our past karma governs the overall results we will experience. Even riding with the wind, you may have a flat tire. Even while sailing with the wind, a shark may jump out of the water and land on your boat. Even if you sow seeds in the best possible season, you may lose the crop to insects or drought etc.
So how does it work? What we are doing by selecting an auspicious time is aligning our actions with the energies of the Universe. That choice is ours. It is our Free Will. Specific planetary alignments indicate “good” or “bad” conditions for doing specific things. Some alignments are good for spiritual work and others are more conducive for material pursuits, undertaking journeys etc.
Now how do we select the auspicious time? Some people rely on their intuition, some get guidance from a Guru and some go to an astrologer. Others just ride the tide.