Based on your comment above, you could say 어제 정말 좋은 날이었어. 그런데 마지막에 좀 슬펐어. But I think it would be better to say more in detail 어제 우리 아들 생일었는데 정말 재밌게 보냈어. 그런데 마지막에 보내야 해서(or 헤어져야 해서) 눈물나고 슬펐어. Don't be so sad... You can see him again in a couple of days.

I actually literally meant that yesterday WAS a good day. I say that all the time. Yesterday was a good day. I had a good day yesterday. (etc) but at the end of the day, it became really depressing. So my reasoning makes more sense, what happened was It was my sons birthday and I spent the day with him but I had to leave at the end of the day (I can't see him for a couple days) and I feel guilty because he cried for a while after I left.. and it was his birthday, which makes it worse..

"Yesterday seemed like a good day...." might be a better way to word it. Then you would think of different grammar to use. May I ask why you're trying to figure out how to say such things? Do you have a pen pal or something?