Generally speaking, you don't ever need to write a spell, do ritual, craft magic, or any other such magical act at any specific time or day. If my need is immediate, then I don't feel the need to wait until the perfect astrological moment. Powerful and effective magic can be done without being an expert in advanced astrology. Holidays occur as “tides of the seasons”, not a single moment that must be seized or lost.
Some people feel that they must perform their ritual at the exact moment that it aligns with some celestial or planetary occurrence. For example, that a Yule celebration must happen on the night of the solstice, at the exact time of the solstice. For example, in the year 2002, the Yule Solstice was at 8:14pm EST on Saturday, December 21. Many people I know had that evening reserved on their calendars for their Yule celebration.
This is actually an excellent time for a celebration, from a practical stand point. But what happens when an event such as this happens at 3:10am on a week day? According to my belief and practice, I consider such events as solstices, equinoxes, and moon phases as tides of energy that can be tapped into and acknowledged, not an exact moment to peak and disappear. If a full moon is on a Thursday night, I might call my circle together on Friday night instead. (For the length of a full moon phase, "3 days before and 3 days after" is a rule of thumb followed by many.)
But that's not to say that it doesn't matter entirely. Aligning your magical working with the tides of energy can assist in the success of your magical work. The whole point is to connect to the universal energy in such a way that you can effect it according to your need. Astrology and related disciplines can help you understand which direction the energy is flowing around you, so you can better tap into it to affect change.
The houses, aspects, and rulership of planets are all astrology terms, and it usually takes several years before one even begins to understand it. Knowing about astrology can aid your work, but it is not necessary for your work. You can gradually pick that up as you go along. Keep reading your almanac and eventually it will begin to make sense to you. Time your celebrations and rituals according to your own personal needs and preferences.
So, if your need is immediate, don't wait. There's no one right-or-wrong time... just different tides of energy flowing in different ways on different days.
However, if your need can be days or weeks in the crafting, then arrange your working for a moon tide that will favor your outcome. You can also do your work when the moon is in the sign (or house) that rules your need. For love, having the moon reside in the sign of either your moon sign or that of your lover's would be nice. (You'd need to know your natal astrology information: ie, the location & position of the planets at your birth). If you want to draw love to you, perform your spell as the moon is waxing (growing larger). If you want to end a painful romantic situation, perform your spell when the moon is waning (getting smaller).
Once you get to know how the energy moves, you can use it in a million different ways.
~Flame RavenHawk