Years ago, when I was growing up, there were three holidays in February. Although we didn't get out of school for it, there was Valentine's Day on the fourteenth. Then there were two holidays in honor of presidents. On the twelfth was Lincoln's Birthday and on the twenty-second was Washington's Birthday. Both of those were celebrated as separate days, honoring two of the greatest Presidents of the United States. Sometime since then the government decided to honor all presidents on one day, regardless whether it was a bad president like Ulysses Grant or a great one like Lincoln or Washington. Because we were already celebrating presidents in February, they chose the third Monday of that month for the new holiday. In essence they combined two holidays into one.
If the representatives of the United States government were inclined to look for a precedent for such an action, and if they were inclined to look in the Bible for that precedent, they could find it in Leviticus 23. When God appointed his feasts for the Jewish people he prescribed two feasts to coincide with each other.
And the LORD spake unto Moses, saying, Speak unto the children of Israel, saying, The fifteenth day of this seventh month shall be the feast of tabernacles for seven days unto the LORD. On the first day shall be an holy convocation: ye shall do no servile work therein. Seven days ye shall offer an offering made by fire unto the LORD: on the eighth day shall be an holy convocation unto you; and ye shall offer an offering made by fire unto the LORD: it is a solemn assembly; and ye shall do no servile work therein. (Lev 23:33-36)
Also in the fifteenth day of the seventh month, when ye have gathered in the fruit of the land, ye shall keep a feast unto the LORD seven days: on the first day shall be a sabbath, and on the eighth day shall be a sabbath. And ye shall take you on the first day the boughs of goodly trees, branches of palm trees, and the boughs of thick trees, and willows of the brook; and ye shall rejoice before the LORD your God seven days. And ye shall keep it a feast unto the LORD seven days in the year. It shall be a statute for ever in your generations: ye shall celebrate it in the seventh month. (Lev 23:39-41)
Many of the Jewish people today don't even realize what God did here. They don't feel cheated out of a holiday because they don't realize God combined two of them on the same days. Most people consider that they are celebrating Succoth, the Feast of Booths, when in synagogue they wave the "four species," the celebration of the Feast of Ingathering. (September 21-27, 2002)
Why did God do this? I don't know. Some would suggest that there was originally a harvest festival at that time, and that God added the Feast of Booths at the same time to celebrate the wandering in the wilderness. If so, then why didn't God just say that the harvest festival (which they wouldn't have celebrated during the exodus) was now a celebration of a different event? Did God, like the United States government, just want to reduce the number of days off for his people? If so, then why add other holidays later?
Having raised the question of why God designates two holidays on the same dates, I am not going to give possible answers. To some this will just be a quaint fact to be filed away. Others may choose to try to figure out an answer for themselves. Whichever type you are, you now have my permission to do with it what you will.