The week_start has been added as an option since commit27b30daea5. This stores as a database parameter the day starting the week, [1.. 7] starting on Monday, while the calendar options expect to get Sunday as 7 % 7 = 0. As a result the week number would be shifted. Commitc72403d2basolved the issue, this commit adds tests for it. opw 1916666 closes odoo/odoo#29838