From Numbers  20

Aaron dies

The Israelites traveled again. They left Kadesh and arrived at Mount Hor.
[22] And the children of Israel, even the whole congregation, journeyed from Kadesh, and came unto mount Hor.


God talked to Moses and Aaron. God said that Aaron would die here at Mount Hor.  Moses and Aaron cannot enter Canaan.  God was angry about Moses hitting the rock at Meribah.  Moses and Aaron were being punished because they did not obey God.
[23] And the LORD spake unto Moses and Aaron in mount Hor, by the coast of the land of Edom, saying,
[24] Aaron shall be gathered unto his people: for he shall not enter into the land which I have given unto the children of Israel, because ye rebelled against my word at the water of Meribah.


Moses, Aaron, and Eleazar should go up on top of Mount Hor. They should take the priest's clothes off Aaron. They should put the priest's clothes on Eleazar. Aaron will die. Eleazar will be the new priest. 
[25] Take Aaron and Eleazar his son, and bring them up unto mount Hor:
[26] And strip Aaron of his garments, and put them upon Eleazar his son: and Aaron shall be gathered unto his people, and shall die there.


Moses, Aaron, and Eleazar did what God told them to do. They went up on top of Mount Hor. All of the other Israelites watched them go up.
[27] And Moses did as the LORD commanded: and they went up into mount Hor in the sight of all the congregation.


Moses took the priest's clothes off Aaron. He put the clothes on Eleazar. Aaron died there on top of the mountain. Then Moses and Eleazar came back down.
[28] And Moses stripped Aaron of his garments, and put them upon Eleazar his son; and Aaron died there in the top of the mount: and Moses and Eleazar came down from the mount.


All of the Israelites were sad that Aaron died. They mourned for him (cried and felt sad) for 30 days. 
[29] And when all the congregation saw that Aaron was dead, they mourned for Aaron thirty days, even all the house of Israel.