What day does the second week of October start?

That depends on up to three things, (a) whether you are referring to calendar weeks or simply groups of seven days, (b) what rule you use to determine which is the first week, and (c) to which year are you referring. Some would argue that it could also depend on (d) what day you believe to be the first day of the week.

If you refer to random groups of seven days, the second week of October begins on October 8. If, however, you refer to calendar weeks, it gets complicated.

The most common ways of numbering the weeks include considering the first week to be...

• the week that includes the first day,
• the week that includes the first Sunday,
• the week that includes the first Monday, or
• the week that includes the first business day.

If the first week of October is that week that includes October 1, the second week starts on the first Sunday after October 1. If the first week is the week that includes the first Sunday, the second week starts on the Sunday that falls after October 7 and before October 15. If the first week is the week that includes the first Monday, the second week starts on the Sunday that falls after October 6 and before October 14. If the first week is the week that includes the first business day, or if you are referring to business weeks or work weeks, then it depends on what country you are in, since every country has a different holiday schedule.