The word सन्ध्या (saMdhyA) means the point of juncture or union in Sanskrit. In Sanskrit grammar, the euphonic combination of words is called सन्धि (saMdhi), and denotes the union of two words. But सन्ध्या (saMdhyA) has several other meanings and is a concept of great significance for the spiritual aspirant. The union of jivAtman (individual soul) and the paramATman (Cosmic Consciousness) is of course the ultimate सन्ध्या (saMdhyA) that is the goal of spiritual practice. However, the ancients realized that by seeing this ultimate सन्ध्या (saMdhyA) in many other types of सन्ध्या (saMdhyA), the human being could be constantly reminded of the ultimate purpose of life. Further along the path of sAdhanA the aspirant seeks to have constant awareness of the object of meditation, the ancients being aware of this fact had devised extremely ingenious ways of using natural phenomena to reinforce this constant awareness.
So we have another meaning of सन्ध्या (saMdhyA), which is the twilight. We should not forget that twilight simply means the time when both lights--that of the sun and the moon--are visible. Although more commonly used for the evening sunset, the morning sunrise is also a time of twilight (see Monier-Williams who uses it in this broader sense). I have a recollection of SwAmi RAma calling सन्ध्या (saMdhyA) the 'wedding of the sun and the moon' in one of his lectures or books. This union is a very special time for Hindus as it is the time for the daily prayer called saMdhyAvandanam. This is a beautiful prayer and its true meaning and significance are worth understanding. Apart from the external सन्ध्या (saMdhyA), there is a lesser known internal सन्ध्या (saMdhyA). The serious student of Yoga should be aware of this esoteric internal सन्ध्या (saMdhyA), and should practice to have direct experience of it.
In the HaTha Yoga PradIpika, SvAtmArAma notes that the sun and the moon should be united. Just as there is a sun and moon in the Universe (macrocosm), there is a sun and moon in the human being (microcosm). Just as the sun sUrya is called the right eye of Brahman, similarly the nAdi associated with the right nostril in the human being is called the solar nAdi, or piNgala (the tawny one). Similarly, corresponding to the Moon, the Ida nAdi connected to the left nostril is called the lunar channel. When these two are united through careful practice of pranAyAma and the jalandhAra bandha and mUla bandha, a great joy ensues. Is it not a surprise then that we are joyous when we see the twilight at sunrise or sunset, when the sun and moon are in the sky at the same time? When the internal सन्ध्या (saMdhyA) is experienced, prANa starts to flow in the central channel or sushUmna. This is the cause of the great joy, for as SwAmi RAma notes "sukha manA sushUmna" (happy mind is sushUmna).
The other important सन्ध्या (saMdhyA) happens at time of birth and death: manifestation and resorption of physical form. The transition from life to death also reminds us of Brahman. In fact, SwAmi RAma refers to the shmAshAna vairAgyam (the non-attachment that people feel at the cremation ground). This is not unusual when understood in the greater context of सन्ध्या (saMdhyA). We have been reminded of our true Source again, at the times when we confront birth and death.
There is still another extremely important सन्ध्या (saMdhyA) that the aspirant should be aware of in advanced practice. This is the सन्ध्या (saMdhyA) between two thoughts of the mind. It is the interval between thoughts that leads us to Brahman, for this is the amanaska (no-mind) state that SvAtmArAma refers to in the HaTha Yoga PradIpika. This is also the state of mind in Yoga NidrA, and an important pre-requisite to become aware of Consciousness that is pre-mind.
By recognizing all these meanings of सन्ध्या (saMdhyA), we can become aware of His presence in the transitions between opposites, for He is the unifying principle who pervades all duality. We can have a glimpse of that unity when we rapidly transition from one opposite to the other, if we can hold our attention at that transition moment; the सन्ध्या (saMdhyA). This is the significance of the daily practice of saMdhyAvandanam: to serve as a daily reminder of our true relation to the macrocosm and the Source of all Manifestation.