Ever heard about that certain spirits are ideally summoned during certain days and certain hours? Ever caught yourself wondering how to determine that exact time?
But, as a free bonus I’ll recommend the software Magicalc as well. That’s the only software I recommend. Most of the apps out there suck and can’t be relied upon to give you the right hours.
The Lunarium and Magicalc are the best ones I’ve found. On non-Windows systems, I just use a link to that site on my desktop.