You can save your game using the SAVE option. Once you click it, a window will appear asking whether it is okay to overwrite existing data. If you click "YES", there will be another window telling you that you will go back to the title screen. Just click "OK" after that.
Be extra careful if you are playing in the second part of the game (after confession).
If you play in Skip Mode and choose to save your game before finishing your scheduled date, you will get this warning message telling you that you will intentionally forget the date. If you are okay with that (your girlfriend might get mad at you), just click はい (yes) button. Otherwise click いいえ (no) and finish your date before saving.
Once you reach the part after confession, if you turn the power off without saving, you will see that your girlfriend gets mad when you turn the power on again. Sometimes, the background on the left screen will show the word "PENALTY" all over. If it's the first time, she might just tell you that she's getting mad. But if you do that again, she will ask you to say some words before you can start playing the game.
What she will ask you will be one of these;
Manaka
「愛してる」 "Ai-shi-te-ru" (You have to say this 3 times) (= I love you)
「君を一番愛してる」 "Ki-mi wo i-chi-ban ai-shi-te-ru" (= I love you the most.)
「好き、好き、大好き」 "Su-ki, Su-ki, Dai-su-ki" (= I like you, I like you, I like you so much.)
Rinko
「愛してる」 "Ai-shi-te-ru" (You have to say this 3 times) (="I love you")
「リンコを一番愛してる」 "Rin-ko wo i-chi-ban ai-shi-te-ru" (= I love Rinko the most)
「好き、好き、大好き」 "Su-ki, Su-ki, Dai-su-ki" (= I like you, I like you, I like you so much.)
Nene
「愛してる」 "Ai-shi-te-ru" (You have to say this 3 times) (="I love you")
「ネネ一番愛してる」 "Ne-ne i-chi-ban ai-shi-te-ru" (= I love Nene the most)
「好き、好き、大好き」 "Su-ki, Su-ki, Dai-su-ki" (= I like you, I like you, I like you so much.)