For anything to be obedience it has to be legislated by Allah or His Messenger and it must be in agreement with the Sharee'ah in the following six points:

Its Reason (Sabab)
If Allah is worshipped for a particular reason, then you must be sure that that is the reason, that Allah or His Messenger has given for that particular worship. It must not be a reason that you or some other human being come up with. Based on this principle, therefore, if regular virtuous acts like Prayer, Fasting, Dua'a (Supplication), Dhikr (remembrance of Allah), Recitation of the Qur'an are done for the wrong reason i.e. any reason that had not been established by Allah or His Messenger, then that action will not be valid and therefore not accepted by Allah.
The following are some examples of this: many Muslims in various parts of the world gather together to commemorate "Maulid an Nabbie" (the birth of the Prophet), "Al- Israa wal Mir'raaj" (the night of the Prophet's journey to the heavens), the night of the 15th of Shaabaan, etc. they come together at these times and make optional prayers, recite Qur'an, do Dhikr, give speeches etc.
However, a close analysis would show us that these commemorations are invalid, because the reason for establishing them were not established by either Allah or His Messenger nor practiced by the companions i.e. there is no reason for such celebrations because they are not sanctioned by Allah's Sharee'ah.
If we are commemorating such events, then definitely it must have some basis in the Deen (Religion) because Allah said: "This day have I perfected for you your Deen, and completed my favours upon you, and pleased for you as deen Islam." (Qur'an 5:3)
If anything is completed, does it need any additions? Does it need any subtractions? Does it need anything to be altered in it? The answer to all three of these question is no.
Therefore, all the above celebrations have to be proven in either the Qur'an or the authentic Hadeeth of the Messenger otherwise they would be additions to the "deen" that Allah completed over 1400 years ago with the passing away of Muhammad (peace be on him). Now, if it is not found therein, would it be correct for us to claim that it is a good action? Definitely not.
However, if we continue these practices due to stubbornness or because it is what our forefathers did or because of what some 'Shaikh' said, then what we are implying from our actions is that Allah's religion (Islam) is not complete. Therefore it follows that Allah (Glory be to Him) is lying, because He said that He completed for us our "deen". In addition we witness that Allah is above all imperfections and above lying.
Looking at this from the point of the Messenger (PBUH), is there any proof that he celebrated these events? So, did he not know that this was a good reason for celebration? If we say that he did not know, then we are accusing him of ignorance. If it is said that he knew but he did not inform us, he is now being accused of cheating, and the Messenger of Allah is above both of these evil traits.
He (PBUH) himself said: "Why do the people refrain from doing something that I did? By Allah, I know Allah more than they (do), and I am more fearful of Allah than them." [Bukhari]
So this negate the claim of those who might feel that he does not know. As for the claim of those who said he knew but did not inform us, let them listen to the following hadeeth, he (PBUH) said: "There is nothing that will take you closer to Allah and away from the fire except that I have commanded you to do it...."
Other actions that also fall in this category are the following:: Qur'anic reading for the dead 40 days after death or on a yearly basis, Qur'anic reading for any regular event in life like acquiring a new car or house , a birthday, a raise at our job, etc. Definitely if we do not find something established in the religion of Allah, it is better to leave it out and not to dot it.

2. Type or Kind (Jins)
Allah must be worshipped with the type or (jins) of a thing if that is what is prescribed by Him or his Messenger (PBUH). For instance, Allah's Messenger had commanded that food items be given for Zakat-ul-Fitr (zakat given after the fast of Ramadhan). However, it is now a regular practise to give this Zakat in monetary value. Because of this some of the recipients reject if they are given food and demand money instead.
Giving money instead of food does not agree with the Sharee'ah in the type that is prescribed by the Lawgiver thus making it invalid according to most Imams and scholars.

3. Way or Method (Kaifiyyah)
Allah must be worshipped in the way he prescribed in His Book or on the tongue of His Messenger (PBUH). We must constantly praise Allah to show how grateful we are because he did not leave us to figure out what methods to use to worship Him. All the different acts of worship had been described in details for our benefit. Based on this principle, it follows that no one can come along and tell us how to worship Allah unless he has proof to do so, regardless of his position and\or knowledge, may he be a Sahabi (Companion of the Prophet), an Imam, a walee or whatever title he holds.
For instance, the Messenger of Allah (PBUH) commanded us relative to the prayer: "Pray as your see me pray". We know that prayer has a prescribed format which follow a prescribed order set by Allah and His Messenger, viz., opening takbeer (saying Allahu Akbar) to start the prayer, Qiyaam (Standing for recitation), Rukoo' (Bowing), Standing after bowing, Sajda (Prostrating) etc.
Now if anyone decides to perform his Salat and start from the last act first like starting with Sajda, then standing, then make Rukoo', then stand to recite the Qur'an and then end with the Takbeer, would this prayer be accepted by Allah? Obviously not and that is because it violates the principle of the method which was prescribed by Allah and His Messenger (PBUH). The same applies to any other act of worship that has prescribed method like Dhikr, Du'aah, etc.

4. Amount (Qadr)
Allah has to be worshipped based on the amount He or His Messenger prescribes. There are certain acts of worship that had been prescribed by amount like five times daily prayer, 2 raka'ah (cycles) for the Fajr Prayer, 4 raka'at for Dhuhr, 'Asr and 'Eshaa, and 3 raka'at for Maghrib. If anyone tries to increase a raka'at or decrease a raka'ah without a reason prescribed by the Sharee'ah, then it will not be accepted from him.
The same principle can be applied form any acts of worship like zakat which has a prescribed amount of 2.5%. Fasting in Ramadhan is for a prescribed number of Days, if this is increased or decreased, then it would not be accepted. Anyone who does this knowing the reality would be subjecting himself to Allah's wrath.

5. Time (Zaman)
There are other acts of worship that can only be done at prescribed times. If they are done in any other time that is not approved by the Sharee'ah then they would not be accepted.
The prayer is a very good reference for this. Allah said: "Indeed the Salat is enjoined on the believers at prescribed times." [Nisaa 4:103]
Now if someone wants to pray the Five compulsory Salat but he decides that because it would be difficult to do during the day, so he will combine them all instead at an appropriate time, would these prayers be accepted? Definitely not.

6. Place (Makan)
Place is another thing for some aspects of worship that had been already established by Allah and His Messenger (PBUH). For instance, can anyone circumambulate (make Tawaaf) around another building other than the Ka'aba or can he\she stand on any other place than Arafat and say it is like standing at Arafat during the Hajj? Definitely not.
These various principles have been discussed to give us guidelines for worship. If we are really sincere about worshipping Allah then, for everything we do, we should look to these six principles and apply them.
If after applying them we find that everything is fine, then we can conclude that particular act is an act of worship and we should hasten to it. If not then we should avoid it.
[Excerpts from: Hudaa]