Pradoṣa(m) means Transition from Light to Darkness. You read it right, it is from the transient (light) to eternal (darkness). Light always needs an external source, darkness doesn't center on any source, it is prevalent all over by default. This transition from light to dark is happening all the time at various levels in and around us.

In a day, during sunset it is pradoṣa kalam.

In a week, saturday sunset is pradoṣa time.

In a month, sunset of the 13th waning lunar day (trayodashi) of waning phase (தேய்பிறை) is pradoṣa kaal. This is what is commonly referred as pradoṣa and special rituals are performed during sunset (the daily Pradoṣa time) on this day. When the weekly pradoṣa (saturday) coincides with this (சனி ப்ரதோஷம்), it gains more significance.

In a year, Maha Shivaratri, 13th night from full-moon in the lunar month of Phalguni/Magha/மாசி by the end of winter. Usually occurs in february, it is day before no moon day. Again, when the weekly pradoṣam of saturday coincides with this (சனி மஹாப்ரதோஷம்), it gains more significance.

Within the day, every time the dominant breath shifts from right to left nostril, it is a pradosha time for that individual. During all saturday, monday and thursdays-of-waxing-phase-of-moon (வளர் பிறை), the left nostril will be naturally dominant in a healthy person. Hence, when a monthly pradoṣa happens on monday or saturday, it is given more significance.

Within every breath cycle, begin of the exhalation cycle is the pradoṣa time. With sealed lips, try to generate any sound while exhaling, it unanimously ends up as "om" (ૐ). This sound is considered the most sacred of all universal sounds (நாத ப்ரம்ம).

We've seen the 6 levels of pradoṣam happening here, this is the time scale experientially perceivable to the common human mind. This kind of conjunction keeps happening in many micro and macro levels as well.

So, every exhalation, everytime the dominant breath just shifts to left nostril, everyday sunset, every saturday (if coincides), the 13th night from full-moon day, maha sivaratri day (தேய்பிறை மாசி பிரதோஷம்) are the most significant times to be meditative.

What to do during pradoṣam?

In east, pradoṣam is translated to time-to-burn-sins. So people go to sacred places and do the form of worship they are aware of. In the name of worship, a vast majority follow what some of the enlightened beings used to do. Though they might be doing the same, they are not actually doing the same. It is analogous to a musical genius playing a violin, and an amateur attempting to impersonate the genius. The music produced is not the same. To generate the similar (though and better if not the same) soulful music, the amateur has to rise to the level of genius. Once risen, there is no need to impersonate and any music produced is divine. Same applies to the worship.

For seasoned meditators,there is not much to be said here, they might be spontaneously getting into meditative states during these times.

Practitioners, can choose these times to practice meditation (or) dharana, pranayama bascially everything that leads towards meditation. Depending on the expertise level, state of mind, situation, determination, ability, the practitioner can choose to meditate for a nādī (24 minutes), muhūrta (48 minutes), hōra (~1 hour), 1 yāma (~3 hours).

Art lovers can choose these times to indulge in giving form to their creativity. This is their meditation.

Devotees, I mean adherent ones who are soulfully surrendered to the object-of-devotion can recite their prayers during these hours. Their chances of getting to the higher spiritual state is highly likely during these times.

Tantric practitioners choose this time for their form of worship.

When?

(once a year) People who are very busy with worldly affairs and can only do a bare minimum for spiritual evolution can attempt their form of worship on the maha sivaratri night. the day before no-moon day in the month of february.

(once a month) Others who can afford a little more time can do it during sunset on the pradoṣam of the waning-moon (தேய்பிறை பிரதோஷம்), i.e, the day before no-moon day.

(once a fortnight) sunset of every 13rd night from both no moon and full moon day.

(once a week) every Saturday during sunset

(once a day) everyday during sunset.

(once every few hours) every time breaths shifts from right to left nostril.

(all the time) every exhalation.