Why is is said. "look east at dawn on the spring equinox to see the zodiacal constellations"?! why on the spring equinox?! why cant i view the constellations on ANY normal day?!
Well that's not a saying I've particularly heard but the only immediate answe I can think of is that at the equinox the Ascendant (the point where the horizon intersects with the ecliptic) is due East at sunrise. This is true for both equinoxes but is not true of any other day of the year, as the Ascendant varies over the year between somewhat North of East and somewhat South of East (the amount of displacement from due East depends on laititude)
In the Northern Hemispehere the Ascendant, at sunrise, moves gradually North of East from Spring Equinox, reaching its most northerly point at the Summer Solsitice, it then moves southwards till at the Autumnal Equinox it is again due East. It then moves South of East reaching it's maximum southerly point at the Winter Solstice (For the Southern Hemisphere simply reverse the seasons).
Incidently on a daily basis, each time that 0 degrees Aries and 0 degrees Libra rise, they are due East - that is the annual cycle of the Sun's rise is repeated on a daily basis with the ascending degrees of the ecliptic.
Now, strictly speaking my answer applies to the ecliptic, the Sun's observed path,. The Zodiac is a band of around 9-9 degrees either side of the ecliptic. The Sun can never be off the ecliptic, so it will always rise due East at the equinoxes, but all the other planets can and do have latitude north or south of the ecliptic, so even on the day of the equinox all of them are likely to rise either a bit North or a bit South of due East
The overwhelming majority of Astrologers, including Vedic, Traditional and Modern, Astrologers, do not use Constellations. For them, the Zodiac circle is divided into twelve equal segments, measured from an arbitrary point (usually the Spring Equinox or a calculated point which keeps the spring equinox in the constellation of Aries (but the other signs don't match up to the constellations with any exactness). This means that even the actual constellations of Aries and Libra are not due East at the time of the Spring and Autumnal equinoxes respectively.