On Linux systems and Unix systems you can use the cal utility to view a calendar. If you type cal, you will see a calendar for the current month.
April 2005 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
If you want a calendar for another month, either in the past or the future, you can specify the month and year with cal mm yyyy. E.g. to view the calendar for February 2005, you could use cal 02 2005.
February 2005 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
You can view the calendar in Julian format with the -j option, i.e. produce a calendar that shows the number of days that have elapsed since the start of the year with January 1 as day one and February 1 as day 32. E.g. cal -j 02 2004 produces a Julian date calendar for February 2004.
February 2004 Sun Mon Tue Wed Thu Fri Sat 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
If you need more features from a calendar display program, you can try the GNU gcal program or the pcal and lcal programs, which can generate postscript and html output.