1. Define a calendar view:

<record id="calendar_project_task" model="ir.ui.view"> 
    <field name="model">project.task</field> 
    <field name="arch" type="xml"> 
        <calendar date_start="date_start" date_stop="date_end" 
                   color="project_id"> 
            <field name="name" /> 
            <field name="user_id" /> 
        </calendar> 
    </field> 
</record> 

2. Add menus and actions using this view. This is left as an exercise for the reader.