How long do you burn candles after someone passes?

The use of a yahrzeit candle is a widely practiced custom, where mourners light a yahrzeit candle that burns for 24 hours, on the anniversary of the death on the Hebrew calendar. The word "yahrzeit" in Yiddish means "anniversary" or more specifically "anniversary of a person's death".

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Regarding this, should you light candles during the day?

It's appropriate to light candles only after dusk, unless it's a particularly rainy or overcast day, and then they may be lit a little earlier in the afternoon. But assuming good weather and sunny skies, candles should not be lit for daytime affairs like brunches, luncheons, or afternoon tea. What happens when the Holy Spirit touches you? If you will allow the Holy Spirit to fill you, to flood you, to be diffused in Him, and you receive what God wants to do in you, then you will realize that you have been healed, delivered, empowered to prosper, have direction, and have His wisdom.

Then, how do you know when you are praying in the spirit?

If you seek to pray in the Holy Spirit, you seek to develop the love character of Jesus Christ, and seek to conform your prayers to desires that are not lustful, but rather, desires that develop your capacity for love. This means desire for professions, spouses, children, material possessions etc. Can you pray with your mind? You can pray mentally through your thoughts: "'The worshipper must pray with a detached spirit, concentrated attention, in unconditional surrender of the will to God, and spiritual passion. ' God knows what is in your mind or heart but its not considered as worship unless you speak.