How to train a model to predict a time window than an event will occur on a website
You can use TICK (Telegraf, Influx DB, Chronograf and Kapacitor) stack for the same. You need to install the above mentioned stack on your web server. Chronograph is a visualization tool which gives you information about your server. 

Morgoth is a unsupervised learning algorithm which can help you achieve your purpose. You need to combine Morgoth framework with Kapacitor so that the event is triggered once you have an anomaly