Can you reject Slingshot DLC mission first month and choose it next month?

Slingshot DLC first mission is a bit hard for first month on Classic.

Chryssalid and huge amount of Thin Men with 100% hit poison

So on the screen with options Slingshot/Normal mission(random) I said normal expecting this question to appear next month again.

But it didn't. So should I start from the start to have Slingshot or these missions will appear at random again?