When you cancel your premium ("downgrade"
it won't really cancel it until the last day you paid for.
And, as you found out, from the time you cancel till the time your account actually reverts back to basic you will not be able to buy land (not even on private estates).
You do have a few options:
1) open a support ticket asking to reinstate your premium (easiest)
You'll be able to buy land normally. If you changed your mind about staying premium then there's nothing else for you to do. If you still want to cancel, make a note (or wait for the reminder email to come) to yourself to cancel a day before your renewal date (in your case March 9th you'd cancel).
2) buy land for group instead (more complicated if you're unfamiliar)
Works for both private estate land and mainland (if you're going to use this for mainland you're far better off just getting the premium reinstated): go to World / About Land when you're on the parcel you want to buy and then simply click the "Buy for group"button to buy the land as group-deeded
Judging from your post I'd guess English isn't your native language so if any part of that wasn't clear, please just post and ask again
. (Or point out your native language and with some luck someone who speaks it will come along
)
Putting in a support ticket really is the easiest way to go. Just remember that if you want to cancel later on to wait until the day before it would actually renew.
few days ago i buyed premuim accaunt dident how every think works in premuim so i Downgrade it but now i know how it works but i cant change it to let it stay at premuim now i get this Account Downgrades: Monday, March 10, 2008 now i cant buy land 
then says Account Downgrades: Monday, March 10, 2008