How often should I send a real estate newsletter?

Hello - I am going to send out an email newsletter to past clients with market updates, predictions, etc... I think I have the mental bandwidth to send out 4 newsletters per year, but I don't know when the best time to send them out would be.  Any advice?  My first place to start would be spring/summer/fall/winter, but should I be more strategic or be sure not to hit a specific time?