Does this calendar show planetary retrogrades for Mercury, Venus, and Mars?

Yes. Retrograde tables run through the calendar year and mark when each of these planets turns stationary and begins moving backward through the zodiac, along with the date it turns direct again. The tables sit alongside the monthly grid rather than buried in the bonus pages, so you can check whether a planet is currently retrograde without flipping through the whole calendar looking for the right page.

Does the calendar include Moon void-of-course times, or only Moon phases?

Both. Each month lists the Moon's sign, its phase, and its void-of-course dates and times, not just the New and Full Moon dates. That combination is what lets you use the calendar for actual timing decisions, choosing or avoiding a specific day, rather than just tracking which phase the Moon happens to be in that week without any further detail to act on.

What time zone are the daily aspects and Moon times calculated in?

This series has historically printed its timing data in Eastern Time. If you live in a different zone, convert from the printed Eastern times using your own standard offset, and account for Daylight Saving during the months it applies so a listed time does not end up an hour off from what you actually observe on the day you are checking it against.

How is this different from Llewellyn's Daily Planetary Guide? Which one should I buy?

This wall calendar gives you a monthly overview: major daily aspects, retrogrades, and favorable days at a glance from across the room. The Daily Planetary Guide is a book-format reference with finer-grained daily and even minute-by-minute timing data, better suited to electional work where you need precision rather than a monthly summary. Many astrologers keep both side by side rather than treating them as substitutes for each other.

Can this calendar actually help with picking good days for gardening or scheduling appointments?

Yes, and that use case predates most of the astrology content on the page. The favorable and demanding days marked on the grid come from the same planting-and-timing tradition running through Llewellyn's almanac line, and readers have used prior editions for exactly this purpose for decades, timing garden tasks and appointments around the marked days rather than reading the deeper astrological detail at all.