As one of the few people who is both a believer in magic and a believer in religion, I want to ask about summoning angels or demons. First off, to what purpose would you summon these beings? Second, how does this apply to God (or any other deity that applies to this)? Finally, does one truly "command" these beings? It seems like a ludicrous idea, but then again, I have been proven wrong before. Any clarification would be greatly appreciated.
You summon a spirit for roughly the same reasons you call a person on the phone. It can be for knowledge, protection, assistance, or almost anything. The Christian God may be displeased if you summon Demons, but only likely if you choose to take their side against Him. Most Deities won't care, as long as you're respectful to them. As far as commanding, that is a bad idea usually. Demons are very offended when someone tries to control them, as are Angels. Summoning is about reaching out to a creature in respect and requesting your will. What you put out often comes back with summoning. If you put out kindness, understanding, and respect, you are likely to get it back.
Please excuse my limited experience. But with even that I have found that it is most effective to call certain entities one would for certain circumstances and at certain times for a more effective result. For instance in Wicca I call on a diety like Hecate on the nights of the dark moon for the most satisfaction in protecting myself. Like in Catholic Christianity I know that I may call on Jesus for Healing for the most effectiveness; and respectively call on St Anthony for finding what is lost. Or Mary for motherly and womanly challenges. So I would summon a demon based on what they are known best for or an Angel or Saint, likewise. I have not been able to find anything on when and in what time on day, month, year... to summon demons and angels. And by summoning I mean that I do not myself demand or summon; that is a darker type of magic than I use. However you may call in your spirit guides of that sourt dieties, like more in a request. I am sure a more experienced practitioner can tell you about that. Or look it up online. I believe that we can command the spirit world through our request, since our physical bodies do rule the physical plane and spirits have an "interest" here or they would not be apparant to us. As I have noticed in other responses here on SOM to similar questions: a demand and summons to a spirit, demon or angel may not really be portrayed as an "order" to them, like it may sound; it may be more to them like a communication of urgency.
The purpose of summoning these beings is to either request them to aid you in some way, in the case of angels, or to compel them to assist you in whatever area they have authority in, in the case of demons.
How does this apply to God or other deities? Well, first, you don't summon gods. They tend to not like to be ordered around. But you can call upon them to aid or assist you. As far as the Judeo-Christian God, It is well documented in religious texts that Solomon was given the knowledge of commanding demons, and Jesus claimed to be, at one point, more powerful than Solomon. If you are interested in that aspect, the Key of Solomon and the books associated with it are of particular interest. Start with the Grimoire of Armandel.
How one commands them is usually by invoking those beings who rule over the entity you are working with. Which, in theory, means you have established a working relationship with these ruling entities. It also helps if you have experience with assuming godforms so that you can command not through your authority but through the authority of that ruling entity. You should be careful in trying this however. If you assume the wrong godform it can have some serious psychological effects.