also look here 
https://www.odoo.com/forum/help-1/question/how-to-display-the-birthday-of-a-customer-in-calendar-solved-67467

1. Activate the developer mode.
2. Go to sales->customers.
3. Select a customer.
4. In the debug view select manage views
5. Select res.partner.form and click edit
6. Choose under which tab you would like to see the birthday displayed, and click on the + button
7. Select birthdate from the pop up and click update
8. Save and close

please note that this is a text field in the database and not a date field. So getting to display in a calendar will not work, neither will auto emailing etc. This will require changes to the base field.

I did this in my module and it works:

<record id="aaaa_id" model="ir.ui.view">
            <field name="name">calendar_aaaa</field>
            <field name="model">res.partner</field>
            <field name="type">calendar</field>
            <field name="arch" type="xml">
                <calendar string="Calendar" date_start="birthday" color="name_of _the_customer">
                    <field name="name_of_the_customer"/>
                </calendar>
            </field>
        </record>