Monday hour one and social gatherings

I have been using my calendar and the Monday Hour One principles to plan things in advance for a while. A close social group of mine doesn’t tend to plan in advance and will create spontaneous gatherings that evening or one night before. This means I usually miss out on opportunities to socialize with them, which has led to me feeling less connected to them. Also, they have started not to invite me to last minute gatherings because they know I probably have other commitments and can’t make it at short notice. However, I really see the value of planning and the results I am getting from planning things in advance this way. It’s also hard to predict when they may decide to plan a spontaneous event, I haven’t noticed a pattern in terms of weekends/weekdays etc.

How do I use the Monday hour one concept to allow for these spontaneous events?