the whole month; an entire month
Emphasizes a full, uninterrupted month; often used with durations or time spans.
It didn't rain for a whole month.
He was hospitalized for an entire month.
Simply means 'one month'; 丸一ヶ月 adds emphasis on the completeness or entirety of that month.
Standard kanji spelling using ヶ (small ke) for the counter.
Variant using ヵ (small ka).
Variant using full-size カ.
Variant using hiragana か.
Formal variant using 箇; less common in everyday writing.
Variant using full-size ケ; may be seen in older texts.
Compound of 丸 (maru, 'whole, complete') + 一 (ichi, 'one') + ヶ月 (kagetsu, counter for months). The ヶ is a simplified form of 箇 (ka), a counter word.