According to international standard ISO 8601, Monday is the first day of the week. Although this is the international standard, several countries, including the United States, Canada, Japan and Brazil consider Sunday as the start of the week. The default setting in Sirvoy follows what is considered the first day of the week in the country where your business (Sirvoy account) is located.
You can manually change the first day of week for the calendar under Settings -> Sirvoy account -> Localization by switching “Week starts on Sunday” to “ON”.