From 4th to second position just for one day
 

After 3 month of link building work my website jumped from 4th to 2nd position in google serp with the main keyphrase.
The satisfaction lasted just for one day, then my website falled down again to the 4th position.

My website ranked 4th for 6 month and this was the first time I saw it in second position.
Do you think this was a good sign for a better ranking or just a one day stand?

I think you should monitor the serp for a while in order to solidify the rank. After the recent google update, there may have been a shift in serp and some sites could have been penalized thus boosting your rank temporarily.