I have had lunch ,it's the hottest day,I have had so far,这两句

have lunch 是一个词组,意思是吃午饭.词组中的 have 是“吃饭”的意思,是一个实义动词.
现在完成时的规则,have/has +动词的过去分词.吃过午饭了,就是：have had lunch.
It's the hottest day I have had 中,也是.前面的主句部分是一个系动词+表语的“系表结构”,day 后面带一个定语从句：（that）I have had so far.have 是现在完成时的助动词,是构造完成时的一个“帮手”,后面的 had,是 have 的过去分词,意思就是“有”,“经历过”的意思.全句意思：我曾经经历过的最热的一天.