Similar words: by day, the day before yesterday, mayday, daybed, payday, heyday, daybreak, everyday. Meaning: adv. gradually and progressively.
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1. Water dropping day by day wears the hardest rock away. 
2. Things go on day by day.
3. Activists are increasing in number day by day.
4. Day by day the structure heightened.
5. We will muddle through and just play it day by day.
6. Day by day he became weaker.
7. Day by day the disappointed lover peaked and pined.
8. Day by day the situation is improving.
9. They're improving day by day.
10. Competition is getting hotter day by day.
11. Day by day he grew weaker.
12. Day by day she learnt more about her job.
13. The old man's health dwindled slowly day by day.
14. Her health was improving day by day.
15. Day by day his condition improved.
16. He amends day by day.
17. I live for the moment, day by day,(sentencedict.com) not for the past.
18. Marion and I are living day by day.
19. This bid changes day by day.
20. She gets weaker day by day.
21. The army... became larger day by day.
22. Day by day Jeffrey began to feel better.
23. My intense dislike for him seemed to grow day by day.
24. By simply following the day by day eating,[http://Sentencedict.com] exercise and positive thinking programme that is detailed in this book.
25. Once the horse understands what is required of him, he should enjoy his work and improve day by day.
26. He can sense the options as they come and go, day by day.
27. Very often bream have remarkably fixed movements and follow the same watery paths day by day.
28. That generates an attitude of formal control, instead of the day by day probing and checking that should be practised.
29. Y., the investigation is moving forward the same way: day by day.
30. On the one hand, a growing, needy world population eliminates nature day by day.
More similar words: by day, the day before yesterday, mayday, daybed, payday, heyday, daybreak, everyday, rainy day, every day, day in and day out, day after day, day-to-day, play by ear, sell-by date, day, days, d-day, today, all day, one day, day off, v-j day, friday, nowaday, midday, sunday, monday, daycare, day care.