月
tsuki
One waxing and waning is what 'a month' means
The picture A moon in the night sky fills out little by little into a round disc, then thins away again. 月 lights the night and counts months; 日 lights the day and counts days.
Where the picture shows up in 月
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the moon in the sky
今夜は月がきれいですね。
kon'ya wa tsuki ga kirei desu ne
The moon is beautiful tonight.
A round moon hangs in a dark sky and a cloud drifts slowly across in front of it
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once per cycle of filling and thinning
家賃は毎月払います。
yachin wa maitsuki haraimasu
I pay the rent every month.
Each time a calendar page is turned, money goes into an envelope and is handed over again
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the next bundle
来月、引っ越します。
raigetsu, hikkoshimasu
I'm moving next month.
The last calendar page is flipped and a circle is drawn on the first square of the next one
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bundles as a counted unit
三か月分の薬をもらいました。
san-kagetsu-bun no kusuri o moraimashita
I got three months' worth of medicine.
Three bundles of medicine packets sit side by side on the desk
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the moon day of the week
月曜日に電話します。
getsuyōbi ni denwa shimasu
I'll call you on Monday.
One square of the calendar is stamped with the moon character and a little telephone drawn inside it
All of it in one picture
月 is one moon filling and thinning. Hang it in the night sky and it is the 月 you can see; count one full cycle of filling and thinning as a bundle and it becomes the 月 of months. Pay money once per bundle and you get 毎月, monthly; point at the next bundle and you get 来月, next month; count three of them and you get 三か月; stamp the name on the first working square of the week and you get 月曜日, Monday. Behind the boxes on the calendar there is always that moon.