I'm not a native speaker but I googled and found the word flextime.
Judging by the Wikipedia article, you can say
I work flexible hours.
I have a flexible working schedule.
If you want to somehow use the words "weekends" and "holidays" in the sentence, you might say
I have no fixed weekends and holidays, and my working schedule is flexible. It is me who decides when to have a weekend or a holiday.
If you just say
I have no fixed weekends and holidays.
... the listener might misinterpret that as "I have a very demanding job, and cannot be sure that I'll be off-work on a particular weekend or holiday".
If you have no boss, that is, if you are self-employed, you might say just that to explain why you have no rigid work schedule:
I am self-employed, thus my working schedule is free and has no fixed weekends or holidays.
I'm a freelancer, thus the terms "weekends" and "holidays" do not apply to my work routine. I work when I need to, and I take rest when I feel like it.