The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

7891. 'And on the first day there shall be a holy convocation' means that at the beginning all must be together. This is clear from the meaning of 'the first day' as the beginning, that is to say, of the deliverance from those who have molested, and so from damnation; and from the meaning of 'a holy convocation' as the regulation that all must be together. The people were called to convocations in order that the whole of Israel might be assembled together and so represent heaven; for all were divided into tribes, tribes into families, and families into households. Regarding the representation of heaven and the communities there by the tribes, families, and households of the children of Israel, see 7836. Here was the reason why those convocations were called 'holy' and were held at each feast, Leviticus 23:27, 36; Numbers 28:26; 29:1, 7, 12. And the feasts themselves were consequently called 'holy convocations', for all male persons were commanded to be present at them. The fact that the feasts were referred to as 'holy convocations' is clear in Moses,

These are the appointed feasts of Jehovah, which you shall call holy convocations, to present a fire-offering to Jehovah. Leviticus 23:37.

The fact that all male persons were to be present on those occasions is clear in the same author,

Three times in the year all your male persons shall appear before Jehovah your God in the place which He will have chosen - at the feast of unleavened bread, at the feast of weeks, and at the feast of tabernacles. Deuteronomy 16:16.