Travel to and accommodation in Hangzhou

My group of 4 will be arriving in Shanghai on 27 Apr. We plan to travel by high speed train from Shanghai to HZ on 29 Apr and stay there till 1 May. Is it safe to buy train tickets only when I arrive in Shanghai on 27 Apr? As 1 May is a public holiday, will tickets run out? Is it better to buy two-way train tickets at Shanghai-Hongqiao Airport upon our arrival in Shanghai?

Please advise. Thanks.