For progress in life, as per the Brihadaranyaka Upanishad (1/3/28):
"asato mā sad gamaya" = From the non-existent or the unperceived, strive for the existent or the truth.
"tamaso mā jyotir gamaya" = From darkness or ignorance, towards the light.
"mrityor mā amritam gamaya" = From death or the transient, strive for the immortal or the eternal.
Deepaavali itself is the culmination of darkness. Lighting lamps on that day signifies the movement from darkness to light.
The day before Deepaavali is known as Yama Chaturdashi. The 14 phases of the moon (from the new moon to the full moon) represent the 14 worlds (from the material world to the spiritual realm) in living beings. The 14th day of the dark fortnight (Krishna Paksha) is associated with death or Yama. In contrast, the day after Deepaavali is celebrated as Annakuta or Govardhana Puja. Life is sustained by the cow (Go) and food (Anna), representing the immortality opposite to death.
Two days before Deepaavali is marked by the absence of Nirriti (righteous conduct, prosperity). On this day, people purchase durable goods. Two days after Deepaavali is Yama Dwitiya, signifying the bond between siblings. This symbolizes the transition from the non-existent to the existent.
In the annual cycle, Deepaavali night is when the moon is in the Vishaakha constellation. In contrast, the Krittika constellation represents the day. On Kartika Purnima, the moon is in the Krittika constellation.
Krittika signifies scissors (cutting), Vishaakha signifies the point of union of two branches.
These are the points of intersection of two cycles. The Earth's orbital path on the larger sphere is the ecliptic. If the equator is drawn on the same sphere, it would be tilted by approximately 23.5 degrees, the same as the tilt of the Earth's ecliptic. From Earth, it appears as if the Sun is orbiting around us. The ecliptic intersects the equator at two points, which are shown on the globe. The point where it rises above the equator, meaning the Sun is moving north from the equator, is Krittika. From this point, like scissors, two paths diverge. At this point, the Sun's rays are perpendicular to the equator. Opposite to it (180 degrees away), the branches of the two cycles meet, which is the Vishaakha constellation. From the Krittika point, a spherical triangle is formed, hence the calculations start from there - "The face or beginning of the nakshatras is Krittika. That is the star of Agni (the foremost one)" (Taittiriya Brahmana, 1/1/2/1). Krittika is the first. Vishaakha is the best. These are the godly constellations. The ones after Krittika are considered godly and they lie to the south. The ones after Vishaakha are considered Yama constellations, in which the Sun moves away from the north.
(Translated from Shri Arun Kumar Upadhyay's Hindi article).