i had fun last weekend.i liked it very much.this was my week

I had fun last weekend。是完全正确的。因为fun是不可数名词。I had fun last weekend。=I had a good time last weekend。注意当this/that/last/next等和表示时间的名词连用的时候在this/that 前面一定不要加介词。这是英语的习惯。楼上的介词at要去掉。