Slotting new customers into a seasonal newsletter campaign?

Hi peoples

Has anyone got any suggestions for slotting new contacts into a campaign that originally had scheduled emails for different times of the year? I made the mistake of setting up the original campaign with dated emails, thinking that new customers would only receive emails relevant to upcoming dates…but whoooooooooops. If the date I specified in the original campaign had passed and new customers joined in, they got swamped with all of the previous emails in one hit. Nasty.

Any suggestions appreciated, and thankyou in advance.