Coming from the る - Verb 掛（か）ける ‘to suspend’, にかけて conveys that something is happening ‘over a period of time’, ‘from (A) until (B)’, or ‘all through (A)’. This grammar structure is a combination of the 格助詞（かくじょし） (case marking particle)
に, and 掛（か）ける combined with the 接続助詞（せつぞくじょし） (conjunctive particle) て. Literally, the meaning can be thought of as follows:
に - Marks (A) as the location/state.
掛（か）けて - States that something is ‘suspended’ (in an ongoing process) in that location/state.
The ‘something’ that is an ongoing process will usually be indicated by a (B) part of the sentence. This expression will primarily be used with nouns.
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今夜（こんや）から朝（あさ）にかけて大雨（おおあめ）が降（ふ）るでしょう。
It looks like it will rain from tonight through til the morning.
毎年（まいとし）、3月末（さんがつまつ）から5月（がつ）の頭（あたま）にかけて多（おお）くの人（ひと）が引（ひ）っ越（こ）しをします。
Every year, a lot of people move from the end of March through the beginning of May.
にかけて is primarily used in written language, although it appears quite often in the news as well, indicating that some weather pattern is expected to continue for a set period of time.
Occasionally, the 副助詞（ふくじょし） (adverbial particle) は is used after にかけて. This transforms the meaning of ‘all through (A)’, to something similar to ‘limited to within (A)’. Any (B) phrase in these types of sentences will be contrasting (B) with (A), rather than saying (B) is occurring within (A).
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今（いま）は雨（あめ）が降（ふ）っているが、朝（あさ）にかけては晴（は）れるでしょう。
It is raining right now, but it seems like it will clear up during the morning. (Limited to the extent of tomorrow morning)
今月（こんげつ）から来月（らいげつ）にかけては忙（いそが）しい時期（じき）になるので、無理（むり）せずに頑張（がんば）ってください。
Limited from this month through until next month, it is going to be a busy season, so please do your best without overworking yourself.
As we can see by these examples, this is an extension of the standard use of は, which shows that everything before it is a ‘set state’. Therefore ‘(A) は (B)’, means that ‘only (A) defines (A), (B) is a separate entity/discussion’.