For a Christian with any measure of true faith, the idea of taking something away from God's word should be unfathomable. But what about adding to it? Is it possible that the scriptures lack something? Do we perhaps need some sacred tradition to help us understand God's word?
Deuteronomy 4:2 is one of many verses that tell us adding to God's word is just as dangerous as removing from it. Here, the Lord clearly tells Israel that adding to, or removing from His words will inevitably result in disobedience. This isn't just an empty threat. There are multiple times recorded in scripture wherein someone did add to the word of God, resulting in some form of disobedience. In fact, adding to the word of God is one of the first philosophical failures of the human race. Eve told Satan that God had commanded "You shall not eat it, nor shall you touch it, lest you die." (Genesis 3:3), but the real command was "but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die." (Genesis 2:17). There was nothing in the original command about touching it, and so when Eve touched the fruit and did not die, Satan had everything he needed to cast doubt on God's word.
But Eve's example is only implicit. The scripture merely implies that her addition gave an advantage to Satan, but it doesn't explicitly say "For laying aside the commandment of God, you hold the tradition of Eve...". What the Bible does say, however, is "For laying aside the commandment of God, you hold the tradition of men". This quote, from Mark 7:8, is part of an event that is recorded twice in the Gospels, the first being in Matthew 15:1-9 and Mark 7:1-13. Both accounts have several things in common, namely the Pharisees had their own traditions, which they preached as authoritative, and this resulted in their disobedience to God's explicit commands. Specifically, Jesus called them out for elevating giving gifts to God above the command to honour your parents.
And so we see that, just as God warned in Deuteronomy 4:2, adding to God's word usually breeds disobedience to it. Now, that doesn't mean you cannot believe or practice anything outside the Bible. That would be ridiculous. Even this article is, in a sense, an "addition" to the Bible. What it does mean is that everything you do add should be put in its proper place. Additions to scripture are not authoritative, scripture itself is. Therefore, anything that is not based in scripture is, at best, irrelevant.
Hanukkah, the Jewish festival of lights, is an excellent example. Can a Christian celebrate Hanukkah? No matter how many times you read the Bible, you will not find Hanukkah explicitly described. But there is implication that Jesus Himself celebrated it with His people. To be fair, this is an implication. It would be an addition in and of itself to say that Jesus definitely celebrated Hanukkah. But the evidence is there.
Romans 14 especially talks about such traditions as being both legitimate and unnecessary. It is the choice of the individual believer, made between them and God. No one may force you to celebrate what you do not want to celebrate, and no one may forbid you to celebrate what you have purposed in your heart to celebrate. The same is true with food and drink, and a range of other things Paul calls "doubtful disputations".
The long and short of it is what you add to scripture, don't add as scripture. Scripture must always be the sole and sufficient authority. Anything that contradicts scripture, oppose it. Anything that does not oppose it, ensure you do not confuse that with scripture. It is binding on no one but yourself, and even when it comes to your own faith, if your addition causes you to sin, drop it before it drops you.