Rated 5 out of 5 stars

This was a very simple yet very useful addon.

Since Thunderbird 45 / Lightning 4.7 versions, the addon is no more compatible (if Show Calendar Week addon is enabled, no calendar entry can be displayed in the month view of Lightning).
Since Lightning 4.7, the week date display is integrated, so this addon is no more needed.

This review is for a previous version of the add-on (0.3).