Both are oks, but if you want to be very very VERY strict, then "esta primavera" is better, but a lot of people in my country use "en esta primavera". Regards.

muchas gracias, Javi! :)

Hi, im spanish, and we say "Esta primavera" ("esta primavera fue de las mas frias de hace muchos años") "En esta primavera" sounds to me more poetical than practical when talking spanish hope it helps ;)

thank you :)

Without a context in difficult to say but generally as Jorge said both are correct, the difference is not that big between the two of them. Saludos :-)