I have always believed in a spring rapture. As you mentioned the text in The Song of Solomon concerning the rapture or catching away of the bride happens in the spring. Also there are (in my opinion) rapture verses that appear in Isaiah 18. I posted my thoughts concerning Isaiah 18 and the spring rapture on Doves quite some time ago. I will share them again with you below. The blue text are the verses from Isaiah 18 and below that in black is my interpretation of them. From my perspective God was definitely referring to the rapture in these verses.
Jesus ascended into heaven in the spring into the clouds. I believe it was mid-May. The angels that were there and spoke to the witnesses of this event told them he would return in like manor or in like fashion. Could the rapture date be that simple that He will return on or near the feast of the Ascension.....in the spring??
For me it's definitely a spring rapture.
4 For so the LORD said unto me, I will take my rest, and I will look from my dwelling place like a clear heat in sunshine, [and] like a cloud of dew in the heat of harvest.
So the Lord said to Isaiah--"I will take my rest". To me this means God's (Jesus') work is done. The sin debt He had to pay for our salvation was paid in full on the cross. He has spent approximately 2000 years preparing a place for His Church / Bride as He promised. The fullness of the Church or quantity of Gentile believers has been accomplished. He has Michael the archangel rounding up Satan and all his fallen angels which are about to be ousted from heaven.
So He is resting and looking from a dwelling place. Could this particular dwelling place be the place He will be when the Rapture happens? The next two phrases really blew me away when I understood them. What was He looking at when He looked from His dwelling place? "Like a clear heat in sunshine and like a cloud of dew in the heat of the harvest." In both instances He is talking about something rising. We know from scripture that the dead in Christ will rise first and then the living in Christ will rise second. Now look at the wording He used in the two phrases. "Like clear heat in the sunshine" That's like the heat you see rising from asphalt on a hot day, it appears to be coming from the earth, it rises and then disappears. As will the dead in Christ--- that will rise first. Now the second phrase "Like a cloud of dew in the heat of the harvest" Again we have something rising but this time the dew is already in the air, it just rises and then it disappears. As will be those who have accepted Christ who are alive----they will just disappear.
5 For before the harvest, when the bud is perfect, and the sour grape is ripening in the flower, he shall both cut off the sprigs with pruning hooks, and take away [and] cut down the branches.
Before the main part of God's earthly harvest, when the bud (Church / Bride) is perfect and the sour grape (Jewish people who haven't sweetened to Him yet) is ripening (ripening because they still haven't accepted Christ as their Savior but will do so) in the flower, He will do two things. Cut off the sprigs with pruning hooks and "take away" (Rapture His Church) and cut down the branches. The branches are the people left behind that have not accepted Him.
6 They shall be left together for the mountain birds of prey, and to the beasts of the earth: and the birds of prey shall summer upon them, and all the beasts of the earth shall winter upon them.
These branches, these left behind people will be left together for the mountain birds of prey to summer upon them and the beasts of the earth shall winter upon them. My interpretation of this is that at the same time Christ's Church is going up in the Rapture, Satan and his fallen angels (mountain birds of prey--possibly UFO's) will be cast to earth and will prey upon the left behind people in the summer. Then at some period later the beasts of the earth (demons entombed in the earth as per Rev. 9) will prey upon the people left behind in the winter as well. Notice God brings summer into the picture first as if the rapture happens in the spring and Satan and his goons show up a few weeks later in the summer.