Imagine a world slightly more developed then ours. For example internal combustion engines are becoming obsolete, they have fuel cells and electric cars. They also started using genetic engineering to eliminate certain diseases and even improve humans. For example, parents could agree to test their genes and even alter inseminated egg cell in vitro. Majority of population still doesn't use this technology and procreate in natural way. However, they do have ability to use genetic manipulation on certain diseases even after that person is born (stem cells etc ...) Age of space exploration and colonization is still somewhere in the future, and is not relevant for the story.
Elite of a certain country in this world is a firm believer in Zodiac signs, horoscope and all associated stuff. Country in question is not largest or most powerful (i.e. they are not US, China or Russia) but they are technologically advanced, with technocratic tendencies, resembling some mix between Germany and Japan in our world. They do have some remnants of democratic institutions, but decisions are more and more made by this technocratic elite. However, their grasp on power is not so firm, they mostly rely on good faith of general population, i.e. belief that they are working in benefit of everyone. General population of the country is heterogeneous, but on average they are mostly interested in their little lives and not about some distant greater good.
Anyway, aforementioned elite is firmly convinced that tough times are ahead for entire world (turbulence, famine, shortages, wars ... ) But if they have quality population, it would also be a opportunity for their country to expand and become leading on the planet. To this end, they already experimented with genetic engineering, but some things are missing. They have determined that date of birth and therefore associated Zodiac sign greatly influences person's character. Therefore, they would like to boost certain signs, and almost completely eliminate others. For example, they would like to eliminate Gemini because of their fickle nature, only female Cancers and male Scorpios would be allowed, reduce number of Leos because of their narcissism . At the same time boost Taurus because of their firmness and Capricorn because of their grim determination etc ...
Question is, how would they do it ? Date of birth depends on date of conception. Forced abortions and impregnations are out of the question, their control over general population is more of psychological one. Also, directly explaining idea would made them look foolish, and may alarm their enemies that they expect war. They do have centralized water supply, so this could be used to increase/decrease fertility at certain time. Final goal would be to create certain mating and non-mating seasons. Any ideas are greatly appreciated, and best answer would be one that would explain smooth transition (i.e. boiling the frog) from have kids whenever you like to April is a good month for giving birth but June is not. Some use of mandate and force is allowed, but it must be done with measure, since we don't want too many questions or riots and rebellion.