The ornamental design for a calendar, as shown and described.
FIG. 1 is a front plan view of a calendar made in accordance with the present design;
FIG. 2 is a front plan view of the section of FIG. 1 shown within an environment of a plurality of unclaimed sections; and,
FIG. 3 is a front plan view of a second embodiment of the section of FIG. 1 shown within an environment of a plurality of unclaimed sections.
The broken line showings of additional unclaimed calendars in FIGS. 2 and 3 are for purposes of showing the design in use with a plurality of calendar sections illustrating environment. The broken lines and structures they depict form no part of the claimed design.