diff --git a/addons/calendar/data/mail_template_data.xml b/addons/calendar/data/mail_template_data.xml
index 7c7be87027d..3d99e18abb6 100644
--- a/addons/calendar/data/mail_template_data.xml
+++ b/addons/calendar/data/mail_template_data.xml
@@ -63,7 +63,7 @@
                 <t t-out="format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format='EEEE', lang_code=object.env.lang) or ''">Tuesday</t>
             </div>
             <div style="font-size: 48px; min-height: auto; font-weight: bold; text-align: center; color: #5F5F5F; background-color: #F8F8F8; border: 1px solid #875A7B;">
-                <t t-out="str(object.event_id.start.day) or ''">4</t>
+                <t t-out="format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format='d', lang_code=object.env.lang) or ''">4</t>
             </div>
             <div style='font-size: 12px; text-align: center; font-weight: bold; color: #ffffff; background-color: #875A7B;'>
                 <t t-out="format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format='MMMM y', lang_code=object.env.lang) or ''">May 2021</t>
@@ -208,7 +208,7 @@
                 <t t-out='format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format="EEEE", lang_code=object.env.lang) or ""'>Tuesday</t>
             </div>
             <div style="font-size: 48px; min-height: auto; font-weight: bold; text-align: center; color: #5F5F5F; background-color: #F8F8F8; border: 1px solid #875A7B;">
-                <t t-out="str(object.event_id.start.day) or ''">4</t>
+                <t t-out="format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format='d', lang_code=object.env.lang) or ''">4</t>
             </div>
             <div style='font-size: 12px; text-align: center; font-weight: bold; color: #ffffff; background-color: #875A7B;'>
                 <t t-out='format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format="MMMM y", lang_code=object.env.lang) or ""'>May 2021</t>
@@ -315,7 +315,7 @@
                 <t t-out='format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format="EEEE", lang_code=object.env.lang) or ""'>Tuesday</t>
             </div>
             <div style="font-size: 48px; min-height: auto; font-weight: bold; text-align: center; color: #5F5F5F; background-color: #F8F8F8; border: 1px solid #875A7B;">
-                <t t-out="str(object.event_id.start.day) or ''">4</t>
+                <t t-out="format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format='d', lang_code=object.env.lang) or ''">4</t>
             </div>
             <div style='font-size: 12px; text-align: center; font-weight: bold; color: #ffffff; background-color: #875A7B;'>
                 <t t-out='format_datetime(dt=object.event_id.start, tz=object.mail_tz if not object.event_id.allday else None, dt_format="MMMM y", lang_code=object.env.lang) or ""'>May 2021</t>