Inferring situations is the key to developing situation-aware applications that exploit users’ situations to support the fulfillment of their tasks on the move. In this paper, we take an attempt to reason about users’ various situations from their calendar events, provided the calendar event data represent accurate scheduling of ‘real-world’ events. We show how ontology can be used to infer situations from calendar events by considering both the semantic and temporal aspects of situations. We develop a situation ontology and propose a semantic rule based approach to deducing and abstracting situations from data collected from calendar system. The feasibility and applicability of our approach is demonstrated by developing a prototype mobile phone call interrupt management application that uses user’s situation information for handling incoming phone calls. We further present an empirical evaluation of our approach on a real dataset. Our preliminary results show that our approach has a great potential to infer users’ various situations.