Please accept my thanks for the very pleasant time I spent together with you for last weekend.
谢谢你陪我度过了一个如此愉快的周末!
please accept my thanks for……对于……请接受我诚挚的谢意
I spent together with you 是作the very pleasant time 的后置定语修饰time,也就是与你一起欢度的时光,注意是spent,使用过去式
for 在这儿作介词,可续接时间yesterday啊last week啊等等for 后面不接做的什么事情么，而是接时间?都可以的for doing sth.