Hello

you can edit view_mode field directly from settings-- >action -- >window action add calender with tree,form like ex :tree,form,calender in your module action

and if module is new then just add calender with tree,form in view_mode field in action view of your model then your action should be like this:

                <record id="your_action_id" model="ir.actions.act_window">
                <field name="name">your model</field>
                <field name="res_model">your.model</field>
                <field name="view_type">form</field>
                <field name="view_mode">tree,form,calendar</field>
                </record>

Thanks
Sandeep

Did you add calendar in view_mode in action?

Like this:

`view_mode=form,tree,calendar`

Go to Settings / Actions / Window Actions then search for your action and open from view. Add calendar in View Mode field as shown in image.