Can someone explain the meaning of this word?

I wrote to a friend 

21 martda turkiye´de gidecegim

my sentence was corrected to 21 martda ayinda

ihave not used this word before

I don´t agree with that correction. ´ayında´ is like in the month of

Example: He comes back from Singapore next January.

Gelecek ocak ayında Singapor´dan döner.

My correction would be:

21 mart´ta turkiye´ye gideceğim

Hopefully someone with stronger Turkish can check...

Your sentence must be like this : 21 Mart´ta Türkiye´ye gideceğim.

Altrnatives:

Martın 21´inde Türkiye´ye gideceğim.

Mart ayının 21´inde Türkiye´ye gideceğim.

 you both for your corrections which i understand.

Gokuyum...what does ayinin mean in your sentence?

The sentence can be translated to English literally like this: I will go to Turkey in march month´s 21. 

Mart ayı is march month in Turkish unlike month march in English.

Yardimin icin tesekkur ederim. Anladim