SharePoint Rhythm of Business Calendar Form Modification

You could edit new form through SharePoint designer 2013.

1.Enable custom script in the SharePoint site collection.

$AdminSiteURL="https://tenant-admin.sharepoint.com"
$SiteURL="https://tenant.sharepoint.com/sites/emilytest" 
Connect-SPOService -URL $AdminSiteURL
Set-SPOSite $SiteURL -DenyAddAndCustomizePages $False

2.Download and install the SharePoint designer 2013.

3.Open the SharePoint calendar list in the SharePoint designer 2013 -> Forms section -> Customize forms listed under here.

Reference:

https://www.codesharepoint.com/sharepoint-tutorial/how-to-customize-sharepoint-classic-forms

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Here is another way to achieve this.

1.Go to List Settings -> Advanced Settings -> Set "Allow management of content types?" as Yes.

2.In the List Settings -> Content Types section -> Click Event content type -> Set Location, Recurrence and Description as Hidden.

3.Go to the calendar list -> List Tab -> Form Web Parts drop down and select the "New Form" -> Add a Content Editor web part -> Insert following codes into "Edit Source" of the Content Editor web part.

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js" type="text/javascript"></script> 
<script type="text/javascript">
$(function(){ $('#ctl00_ctl33_g_13857511_a957_4788_aa6d_17252e661cd4_ctl00_ctl05_ctl05_ctl00_ctl00_ctl05_ctl00_ctl00_AllDayEventField').attr("checked","true");
});
</script>

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