We have an Ashoka tree growing near to my home, and these pictures were taken during last April when it flowered. The flowering season is from March to May.
According to the Hindu epic ( 'ithihasa') Ramayana, Sita Devi had sit under an Ashoka tree while at Lanka and till the end of Rama - Ravana war, Sita Devi remained there itself under the tree. The place was called 'Ashokavani', 'vani' means garden. It is mentioned in 'Sundarakandam' of 'Ezhuthachan Ramayanam', the version popular in Kerala. There the tree is called as 'Shimshipa' its another name. I myself had read it.