Charlotte Bronte knew full well the implications of placing the initial proposal of marriage of Rochester to Jane on Midsummer's Eve (June 20th.) The festival of Midsummer was related to pre-Christian fertility rites (Britannica), and was a time for rejoicing and enjoying the sunny outdoors of northern countries. On this night Jane and Rochester symbolically walk in the sunset orchard and laurel avenue, and sit under a great horse-chestnut tree, and come to the dramatic conclusion that they love each other and will marry. This buccolic scene of two people coming together in nature fits neatly with the ancient meaning of that time of year.
The choice of time of year was also practical -- in late June in England the nights are warm, and since it is the summer solstice the evenings and twilights are at their longest, with the sun not setting until quite late in the evening. This gave Rochester and Jane a good long time in the garden together, in comfortable conditions in the beautiful fading light. However, at that time of year sudden thunderstorms are possible, which chase Rochester and Jane indoors (and bode ill for the future of their relationship.)
Sources: Midsummer’s Eve." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 08 Jan. 2009 <http://www.britannica.com/EBchecked/topic/381655/Midsummers-Eve>. ,Gutenberg Etext of Jane Eyre http://www.gutenberg.org/files/1260/1260-h/1260-h.htm