Just a few comments here to nuance a bit the answers given above : According the ISO 8601, the first week of the year is the week containing the first Thursday.

The logic behind this is : The first week of a month is the first week where you have a majority of days of the new month vs the previous month.

But that logic varies from company to company: Some companies consider 5 days per week. => The first week of a month is the first week with a Wednesday. (2 days old month, 3 days new month)

Some companies consider 7 days per week. => The first week of a month is the first week with a Thursday. (3 days old month, 4 days new month)

Some companies consider the 1st of the month. => The first week of a month is the first week with the 1st