The main benefit of using a fishing calendar is knowing when and what time to fish in a particular area and get good results. Fishing calendars such as those available from Farmers' Almanac, Bassmaster and In-Fisherman show the days of the month and times of the day that are best for fishing.
The Farmers' Almanac fishing calendar has a simple setup listing each day of the month and a rating between "poor" and "best." It also lists what time of the day is best for fishing, even if the overall rating for the day is poor. A "poor" rating means that fish may not even bite. A "fair" rating means that a few fish may bite, but it is likely to be difficult to land them. A "good" rating is enough fish to be satisfied, and "best" means a fish every time the line goes in.
In-Fisherman has location-specific results as well as species-specific. The calendar displays in a table format with phases of the moon and best times listed. Days are rated in a color gradient from "could be better" to "great day to fish." Bassmaster has an astro table without a specified location and rates days "poor," "fair," "good" or "excellent."