The "julian day" you seem to mention is just the number of days in the year - ie.,
February 5, 2005 is the 36th day of 2005. In computerese it's called the year-day or day of the year.
What you need is a list of the number of days in each month for a whole year -
with February having either 28 or 29 days, to handle a leap year.
Then, work down the list of months (taking into account which version of February to use) by:
top:
Compare number of "julian" days to length of the month.
If the length of the month is greater than the days left, you have the month name
and the number of days is the day of the month. Exit at this point.
If the number of days is greater than the month, subtract the month, make the next month in the list the current month
(remember the February problem? Is this a leap year, yes or no? Is the month February?).
next, go to top
Now try to change the English into pseudocode.
If the Julian date really is a Julian date, and not a year-day, then you need to know which Julian date system is being used. Days since Jan 1, 1857, days since the current epoch, or days since Dec 31, 4713 BC? And then you have a far tougher problem. And it may include the 'short September 1752' problem if you live in the US.
see:
http://www.dome-igm.com/convers.htm