Island Man: Check out the commentaries on Job 1:5
http://biblehub.com/commentaries/job/1-5.htm
Some agree with you, some allow for a different scenario.
In particular, the Benson Commentary actually comments on the Hebrew:
Job 1:5. When the days of their feasting were gone about — When each of them had had his turn, and there was some considerable interval before their next feasting-time; or, as the Hebrew כי הקופו ימי, chi hikkipu jemee, may be rendered, As the days went about...
Also, consider the rendering in other translation/versions along these lines: "...and when the banquet-days had gone the round Job used to send...." (Byington)
That could indicate that there were multiple days for each banquet. The Revised NWT agrees with this thought on Job 1:5.
After a series of banquet days was complete...
So, it is possible that Job would send and sanctify his sons after each multi-day banquet was completed.
If that was the case, these could still be birthdays. Many commentators agree that the Hebrew term rendered "his day" in Job 1:4 is the same term used in Job 3:1 and must indicate a birthday.