How to calculate week number?

[Horace Ho]

In Date Book, the first week of 1999 starts Jan 3, 1999 (1),
where the first week of 1998 starts on Dec 28, 1997 (2).

So, what's the official rule to calculate the week number
for a particular date?

Thanks
horace

Note:
(1) Jan  4, 1999 if "Week starts" Monday as Preference
(2) Dec 29, 1997 if "Week starts" Monday as Preference